CLEAVAGE OF THE YOLK. 



951 



DEVELOPMENT. 



CHANGES IN THE OVUM. FIRST FORMATION OF THE EMBRYO AND 

 ITS APPENDAGES. 



The first essential change which occurs in the fertilized ovum, is 

 the so-called cleavage or segmentation of the yolk. In the holoblastic 

 Mammalian ovum, the yolk is seen to be agitated by a peculiar move- 

 ment to elongate, contract itself in the middle, and then to divide 

 into two. Each half rapidly undergoes further movement, contrac- 

 tion, and division, so that it now consists of four parts. By subse- 

 quent subdivision, these next form eight, sixteen, thirty-two parts, 



Fig. 121. 



Fig. 121. Changes in the ovum of a Mammalian animal, after fertilization (Allen Thomson), a to , 

 successive stages in the segmentation or cleavage of the yolk, a, yolk still undivided; ft, cleft into two 

 masses, each surrounding a nucleus : c, divided into four; d, non-divided ; e, mulberry-like stage of sub- 

 division, to form the blastoderm. The small nucleated yolk-masses have no distinct envelopes. The ovum 

 has acquired a thicker coat, or chorion. After this, the surface of the yolk again becomes smooth, and 

 composed of nucleated cells, having true envelopes. The diagram f, shows the relations of the embryo 

 and its so-called appendages. The dark curved body is the embryo; beneath it is the open part, leading 

 by the vitelline duct, to the yolk-sac, or future umbilical vesicle. Immediately surrounding the embryo, 

 above it, and at each end, is the sac of the amnion. or true amnion; the dotted line shows the false am- 

 nion. Outside this, is the chorion, cho, with its rudimentary villi. The bladder-like organ, projecting 

 from the hinder end of the embryo, is the allantois. 



and so forth. The first effect of this cleavage, is to transform the 

 yolk into a mulberry-looking mass ; but, after repeated subdivision, 

 the surface again becomes smooth, and uniform or granular, and is 

 composed entirely of an immense number of polyhedral nucleated cells, 

 which form a layer within the vitelline membrane, and constitute the 

 so-called germ-sac of Coste, or blastodermic vesicle of Bischoff. The 



