EMBRYOLOGY. 26 EMBRYOLOGY. ers of cells and below by yolk (see below — ac- count of development of a chick). i i.iMin la Stage. It is necessary next that the digestive cells should come to line a digestive sac, while the sensory and protective cells re- main on the outside of the germ. This is effected in a variety of ways (mentioned more particularly later), the simplest of which is the a stimulus to or 'animated' the egg, and it was long thought that the fluid of the sperm and not the spermatozoon was the 'fertilizing 1 agent; the spermatozoa, indeed, were regarded as parasites. It is now known that the essence of fertilization is the union of two cells, and probably especially the union of their nuclei: for the head of the spermatozoon is composed almost wholly of the , nucleus, and it is especially the head that pen- pushing in of the hlastula wall at one pole, as etrates into the egg. The head i- at first very small compared with the nucleus of the egg-cell, hut it rapid- ly swells with water as it enters the egg, and when the nuclei unite they , about tin- same size. The two nuclei contain the same number of chromosomes (see Cell), so that when they unite the number of chromosome- is doubled in the nucleus, half being derived from the mother and half from the father. Thus ma- ternal and paternal qualities are united in the fertilized egg, and are united in equal amounts. This accounts for the equality of inheritance by the offspring from the two parents. In plants the fusion of nuclei derived from the ovule and from the pollen-grain occurs in a fashion exactly comparable with that of animals. The pollen- in typically contain- two nuclei, of which the second has been seen in some cases to unite with a second nucleus of the ovule — namely, the endosperm nucleus. As a result of this union those structures of the -red that are derived from the endosperm seem to derive their qualities from the t»o parents. This result is most striking when the parents belong to different species, char- acterized by different kinds of endosperm. l'livi.ooiNitii' Origin of Fertilization. The periodical union of two eells is seen even among tli.. lowest animal- and plants. In some cases the union i- a permanent one, followed by rapid cell-division and the production of new individ- ual-. Sometimes, however, the union is a tem- porary one, and is characterized only by the interchange of nuclear material. Ii is a striking fad thai even in the Protozoa a reduction of the .■In- atic material occurs before fertilization. It is clear then that a fertilization process i- our of very great physiological importance to organ- isms. General Sketi a 01 Egg Devei opm s d in Animals. Since the egg i- a single .-ell. ii must divide repeatedlj to give rise to the multicellular body. These early cell-divisions of ii gg con i ii hi,, cleavage, an, I the cell - are called cleav- spheres. After eight or sixteen cells have arisen, a central cavity begins t,> appear -ur- ded bj the-,, cells. The prol cists 1 a. I tl.i- cat ity uniil a hundred or more cells have arisen. The whole embryo at Ho the 'iila-iula.' and the cavity i- the hla-tnla cavit y. The indh idual ! - now a hoi ■ i.-, frequi i eaj t lot "mo i ion in v atei i n i he imple I ■ a i it i i cells, and i I be 1 1 ult of not laden willi yolk. In . unequal segmental ion I he bin i| cell -. and tl>.' hoi irtly filled v ii h I., i ell I ich d - i lie hen' ■ that ha < undergo part ial the cleavage ca Ity It reduced to a tlni.l Hlli bounded a eral EMBRYOLOGY OF THE FOWL. 1. Earliest stage : Surface of the germinal disk of an egg, showing Initial segmentation, which begins at the centre and spreadstoward the periphery, but does not quite reach it ; .r. surrounding food-yolk. 2. First appearance of the •groove' or furrow ,/"). :|. m her furrows if) appear, divid- lngthedisk into four irregular areas, each representing a r iit.-.i mass. 4 Additional furrows (M branch off and int. iv,, ret, dividing the disk into many small central and larger peripheral areas; this process continues, as indi- cated to it,,- beginnings ,,t' Dew furrows (r). The remain- ing figures arc diagrammatic vertical Bections through the germinal part of tl . embrj ,,, to show the method [vanclng growth. 5,6,7,8,9, t". Longitudinal sec- tions, it,,, small (head) end of the egg toward the left. 11. A i ransverse section through the centre of the embryo, rebro-spinal marrow (m), and the chorda dorsalls (cA), above and below theaxls of the spinal col- umn and the downward growth of the body-walls on each ni.1.,; also the beginnings of the alimentary canal (a'). 12. o it i ■ . i i r i .- 1 1 section ,it a more advanced stage. 13. Transverse section after the Enclosure of the alimentary it Longitudinal Bectlon at this stage. Let- tering on proper; a', alimentary canal ; &c, cav- ity holding the liquor amnll and it,,' allantois; :if, folds of the an n; at, allantois ; en, chorda dorsalis; e, embryo: in. cerebri i pinal marrow : pi re pleui , i peritoneal e ; j's, yolk-sac j > lembrane. (After Bal- one might push in the wall of a hollow tennis hall. This i- the ail of 'invagination.' The in- raginated layer pai tlj oi « noil] obliterates the hla-tnla cavity; an, I a nev cavity opening wide- l to the exterior is formed —the 'gastrula' cavity, or primil ive t.,,,.i ca> itj . Thi tage i knov n as the gii I,, 1 tage it corresponds essentially to the a. lull condition of ccelenterates, The outer
