ANIMAL,] REPRODUCTION 419 except in size and phase of cell-life, there is little to be noted here in regard to the development of the group, except in such cases as Katallacta, and Volvocinese, where a primitive attempt towards a more or less temporary multicellular form affords us a first hint of the Metazoan segmentation of the ovum. IHcyemida. In this degenerate and still somewhat problematical intermediate group the asexually produced ovum segments to form an embryo of great simplicity, where a few outer (ectodermal) cells cluster round a single inner (endodermal) one, and where ve thus find the first differentiation of the embryo into the cell-layers constantly exhibited in the succeeding groups, though it is unsettled whether this represent the incipient or the degenerate Metazoan type. Porifcra. In a calcareous Sponge, such as Sycandra, the fertilized amoeboid ovum segments within the parent to form a hollow blastosphere, most of the cells of which are clear and ciliated, and partially cover a number of larger granular cells, the future endoderm and ectoderm respectively. In the free-swimming embryo the latter nutritive cells increase rapidly in size, and, growing ultimately over the locomotor and respiratory ciliated cells, cause them to become invagiuated to form the endoderm. This invaginatiou resulting in the obliteration of the "segmen- tation cavity" and the formation of the "gastrula" stage is perfected as the young Sponge settles down and becomes fixed by processes from the amoeboid ectoderm cells. The cilia of the endoderm disappear ; the blastopore is obliterated ; between ectoderm and endoderm there appears. a third germinal layer or mesoderm, and in it the calcareous spicules of the Sponge are developed. A perforation at the top of the inverted gastrula forms the exhalent aperture ; peripheral inhalent pores also arise ; and this renewed exposure of the endoderm cells to direct contact with the water is followed by their return to the ciliated form. See SPONGES. Ccelentera. The generally equal segmentation of the ovum re- sults as usual in a solid or hollow morula. The formation of the two layers, as in some other groups, may occur either by invagina- tion, as above described, or by a process of separation of inner from outer, termed delamination. In most Ccelentera the tissues which in higher groups are strictly mesodermal originate from the direct differentiation of the two primary layers. The larval form most constant in the Ccelentera is the planula (see HTDROZOA, vol. xii. p. 557). It is ciliated, two-layered, almost cylindrical, with a rudi- mentary digestive cavity, generally closed. For discussion of the alternation of generations so characteristic of many Ccelenterates see HYDROZOA and SEX. Vcrmes. The development of the unjointed Worms is too varied to admit of general summary. In some we find illustration of that mode of gastrula formation (epibolic), so common in higher groups, where, owing to the large size of the food-packed endoderm cells, an actual invagination is impossible, but the ectoderm cells grow round the others and thus come to enclose them as in the more primitive process. In such a typical higher Worm as Lumbricus, the segmentation, which varies in regularity according to the species, results in the formation of a blastosphere with readily distinguishable ectoderm and endoderm ; the invagination of the latter results in a somewhat cylindrical gastrula. The blasto- pore narrows anteriorly to form the permanent mouth, while in some other Chaetopods a similar narrowing in the opposite direction forms not the mouth but the anus. The mesoderm appears very early in the form of posteriorly situated cells on each side of the middle line, and soon two inesoblastic bands are formed extend- ing to the rnouth. These are subsequently divided into blocks or somites, the anterior and posterior walls of two adjacent blocks uniting to form the cross partitions of the adult Worm, while the outer and inner walls form respectively the somatic and splanchnic layers. The ventral nerve cord results from the sinking in and union of two epiblastic thickenings, developed one on each side of a ventral furrow, while the supra-oesophageal ganglion has a distinct origin from a dorsal thickening. The somewhat isolated Sagittct affords good illustration of the formation of the body- cavity from two lateral lobes of the archenteron. The simple enteric cavity normally formed exhibits anteriorly a division into a middle lobe and two side pouches uniting behind in the single cavity. The middle lobe is separated off from lateral and posterior cavities as a blind tube which afterwards forms the alimentary canal ; the side pouches form the body-cavity (see TAPEWORMS, PLANAIUANS, NEMERTINES, ANNELIDA, &c.). Echinodcrmata. The generally uniform segmentation results in a blastosphere, usually elongated in the direction of the axis of invagination. A gastrula is formed by normal invagination, in the course of which amreboid cells are budded off from the two sides of the advancing depression to form the mesodermic musculature and connective tissue of the adult. The blastopore forms the larval anus, which does not always persist ; the larval mouth is per- manent, and is the result of an independent ectodermic invagina- tion meeting the archenteron. The cralom or body-cavity, with its mesodermic lining, arises from a paired or single outgrowth of the archenteron, and the water- vascular vesicle which comes to lie round the oesophagus has a similar origin. For an outline of the nature of the larval forms and the passage of the embryonic organs into those of the adult the reader is referred to ECHINO- DERMATA. Arthropoda.The interesting Pcripatus (see MYRIAPODA, vol. xvii. p. 116) exhibits in its development phenomena, such as that of the elongated blastopore gradually closing from the middle, so as to leave mouth and anus, and that of the surrounding nerve cord, which suggest affinities through the whole animal kingdom from Ccelentera on the one hand to Vertebrata on the other ; but of these discussion is still far from ended. The development of the Insect ovum illustrates what is termed centrolecithal segmentation, where a single uniform layer of superficial cells encloses a nucleated but undivided central yolk mass. The blastoderm becomes thickened ventrally; and along the middle line of the ventral plate there appears a groove which is the seat of the mesoderm formation. The development of the Vertebrate amnion is recalled by the formation of two enveloping membranes from folds of the blastoderm, which arch over and coalesce over the ventral plate, and at the same time spread upwards to cover the dorsal integument from which they are eventually separated. Both disappear through absorption or rupture either before or during hatching. The mesoderm is divided into two lateral bands, and these into hollow somites, which together form the perivisceral cavity. As usual the outer layer of mesoderm follows the ecto- derm, the inner the endoderm, which probably arises from a secondary segmentation of the yolk. The ventral plate extends to a slight extent dorsally both before and behind, and on it the appendages appear as hollow outgrowths of ectoderm and endo- derm. The ventral nerve cord arises from two sunken thickenings of ectoderm, the tracheae as invaginations of the same. The middle section of the alimentary tract is formed from endoderm, the anterior and posterior portions from ectodermic invaginations. For an account of larval metamorphosis, &c. , see INSECTS. Crustacea. In these the segmentation is usually centrolecithal, and results in a uniform blastoderm enclosing a central yolk mass without traces of division. The gastrula stage is represented by an invagination of a patch of the blastoderm, which forms the endoderm, and from which the mesodermic cells seem also to arise. A ventral thickening of ectoderm continuous with the invaginated portion forms the ventral plate on which the embryo is mapped out. Along it the nerve cord appears as a median thickening, and the sense organs have as usual a similar ectodermic origin. The mesoderm appears to form the usual tissues (muscles, heart, &c. ), but has apparently less definiteness of arrangement into layers and somites than is usually the case. The original invagination forms the mesenteron, and the lining endoderm cells absorb the yolk The anterior and posterior portions of the alimentary tract are formed as usual by subsequent independent invaginations. Much less proportionally is known of the organogeny than of the very interesting and suggestive larval forms, for which see CRUSTACEA. Mollusca. The typical Molluscan segmentation is markedly unequal, the disproportion between ectoderm and endoderm cells varying according to the proportion of food-yolk. The gastrula stage is more frequently reached by the growth of the ectoderm cells over the larger and fewer endoderm cells than by any actual invagination. The blastopore may close apparently towards either the oral or the anal extremity, forming mouth or anus or neither. In Ceplwlopoda the segmentation is confined to a germinal disk of formative material as in partially segmenting Vertebrate ova, though there are besides cells from the yolk which underlie the germinal region of the blastoderm and also accompany it in, its growth round the yolk, forming the inner blastodermic layer or yolk membrane. The mesoderm appears at the lips of the blasto- pore, or in an equivalent position ; it forms a complete layer between ectoderm and endoderm, and within it the body cavity is formed usually by a definite splitting into two layers. The invaginated endoderm forms the mesenteron ; and the history of the formation of the anterior and posterior portions of the tract varies with the fate of the blastopore. In the Cephalopoda at least, the nervous system appears not as usual from the ectoderm but from the meso- derm. The characteristic larval organs, the shell gland and the foot, are ectodermic in origin the former arising as a thickened invagination on the posterior and dorsal side of the embryo, and the latter as a ventral protuberance. Further details and larval changes are thoroughly treated under MOLLUSCA. Tunicata. The complete and generally quite equal segmenta- tion results in the formation of a blastosphere with a large segmenta- tion cavity. The cavity is obliterated by a normal invagination, and a typical gastrula is formed. In the formation of the nerve cord from the closure of a medullary groove and of the notochord from a cord of cells on the dorsal wall of the archenteron, in the appear- ance of respiratory slits in the anterior portion of the enteric cavity and of a cerebral eye, and in fact in all the essential features of their development they present the amplest evidence of their Vertebrate affinities and degenerate origin (see TUNICATA).
