EMBRYOLOGY. 27 layer in both the gastrula and the adult cce- lcnterate is called ectoderm or epiblast; the in- ner layer, entoderm or hypoblast. From the ectoderm are developed later by differentiation various organs, such as the nervous system, vari- ous sense organs, feathers, hair, and nails; while from the endoderm arise not only the digestive EMBRYOLOGY. Fig 1C. EMUKYU CHICK FROM ABOVE. A chick, 30-66 hours old after Incubation : »p, area (area pel- lucida) of the germinal disk surrounding the embryo, which has formed about the fine of. I be primitive furrow (we Fig 2); fb, mb. hb, resides "i bhefore, mid. and hind bruin; ov, optic vesi- cle; :tu. p. auditory pit; /mm-, protovertebra ; mc, medullary ranal. in I lie saetal iv^iou ; /</■.,-,, hinder part of the 'primitive streak*; tf, tail-fold of the amnion. (Fig. 14. See legend above.) canal, but all ils outgrowths, such as lungs, liver, etc.; and also the notochord of vertebrates. Germ-Layers. In the higher animals the gas- trula stage is quickly followed by one in which loose cells arc formed that migrate into the blas- tula cavity: in their totality these migratory cells are called mesenchyme. Very often, too, especiallj' in the higher forms, a double layer of cells, lying in contact with the ectoderm and entoderm respectively, is formed between the two primitive layers. This is called the mesoderm. The cavity in the mesoderm is the true body- cavity or caelum. The layers, ectoderm, ento- derm, and mesoderm, are the so-called germ- layers. The theory that they are homologous (i.e. strictly comparable) in all animals where they occur, ami that they always give rise to the same organs, is the germ-layer theory — one that held sway in the last quarter of the nineteenth century. It has been essentially modified and partially abandoned in the face of facts. By the time the germ-layers are established the embryo in many cases, especially of aquatic animals, frees itself from the egg-envelopes and becomes free-living; this constitutes the larval stage. The larva is often very different from the adult in form and habits, and is adapted to its peculiar environment. After living for a longer or shorter period as a larva the embryo under- goes a rapid change of form, called metamor- phosis (q.v. ), which leads to the adult condition. The egg-development of plants differs from the above in the absence of blastula and gastrula cavities, and of membranes; the germ is a -< .1 i<l cell-mass. We shall now consider in more detail the different stages in development The Media in which Development Occurs. In the lower invertebrates, fishes and Amphioxus, the egg develop*s chiefly in the water, into which the ripe egg and sperm are both thrown. In other cases, however, the embryo is especially pro- tected by some means for a certain time during early development. Whether the egg develops Vol. VII.— 3. away from the mother, or in connection with the maternal tissue,, baa a great influence upon de- velopment. In the higher plants the embryo is imbedded in the maternal tissues. Cleavage. The nature of cleavage depends upon the disposition and amount of yolk in the egg. The kinds of cleavage are classified as fol- low a : I. Complete division — holoblastic cleavage. (1.) Alecithal eggs (with little or evenly dif- fused yolk) — cleavage equal. (2.) Telolecithal eggs (with yolk lying toward one pole) — cleavage unequal. II. Partial division — meroblastie cleavage. (1.) Telolecithal eggs — discoidal cleavage. (2.) Centrolecithal eggs (with yolk at centre) — superficial cletn age. Equal cleavage is characteristic of sponges, some coelenterates, many worms, echinoderms, some mollusks, tunieates, Amphioxus, and the higher mammals. Unequal cleavage is char- acteristic of many coelenterates, worms and mol- lusks, the lower fishes and amphibians. Dis- coidal cleavage is found in cephalopod mollusks, elasmobranch and teleost (bony) fishes, reptiles, birds, and the lowest mammals. Superficial cleav- age is found in most arthropods. When the egg- cell is not completely cleft (partial division) the 'cleavage spheres' are not well defined basally, and the later stages are complicated and deviate from the typical conditions described above. Gastrulation Reconsidered. Three principal types of gastrulation are recognized among ani- mals: (a.) Invagination, which, as the typical process, has been already referred to. In partial cleavage the yolk-laden cells cannot he pushed into the small cleavage cavity, and under these circumstances the ectoderm grows over the entoderm and gradually surrounds it completely; this process is known as 'epiboly,' in contrast to the typical embolic invagination that occurs in alecithal eggs. (b.) Delamination. The cells of the one-layered germ divide into a superficial and a deep-lying layer of cells. The latter be- come the entoderm. This type of gastrulation occurs in sponges, various coelenterates, and some of the lower worms, (e.) Tngression. Al one pole of the one-layered germ, cells begin to migrate into the blastula cavity to form the inner layer. This process is found especially in hydroids. The aim of all sorts of gastrulation is the establish- ment of a two-layered germ containing a diges- tive cavity, (d.) Formation of mesoderm. The mesoderm arises in a great number of different ways — sometimes as a pair of pouches or a series of paired pouches from the entoderm, sometimes from primitive 'mesoblast' cells which arise very early; sometimes split off from the other germ-layers. Indeed, the varied origin of the mesoderm, despite the similarity of the or- gans to which it gives rise, i, one of the strong- est bits of evidence of the inadequacy of the cell- layer doctrine. It will not be practicable to go into the de- velopmental history of all the groups. We will, however, consider in some detail the development of the chick. For the development of man. see Emuryology, Human. Development of a Dicotyledonous Plant. The fertilized egs; lies deeply imbedded in the ovary in a sac — the embryo-sac — filled with nu- tritive fluid. The fertilized egg. or oospore, de- velops by means of a series of transverse and
