312 EMBRYONIC DEVELOPMENT 



♦ ■ IV. THE LAID EGG. 



When, observed immediately after they have been laid, the eggs are composed of 

 the following parts : 1st, The yolk's sphere, with the germinative .vesicle in its middle; 

 2d, A membrane surrounding the yolk or vitelline membrane ; 3d, A zone of transpar- 

 ent albumen, and 4th, An external membrane, analogous to the so-called chorion, en- 

 closing and limiting the whole egg (figs. 12 and 13). 



The formation of the albumen and external membrane takes place during the pas- 

 sage of the eggs out from the oviduct. Whether in the oviduct proper, in the coital 

 pouch, or in the cloaca, I am not prepared to say just now, from direct observations, 

 but feel confident that I may solve the question ere long, in the pursuit of my inves- 

 tigations. 



For a short time after the eggs have been laid we still observe a clear space towards 

 one point of their surface, appearing however quite small on account of the increase 

 of the vitelline substance now filling up densely the whole sphere (fig. 13). 



The absolute size of the eggs varies within certain limits ; a few being found one 

 third smaller than the usual size. The form itself is subjected to variations ; generally 

 circular, it sometimes elongates and becomes oblong or oval in some few. 



V. THE DIVISION OF THE VITELLUS. 



The division of the vitellus or yolk commences about twelve hours after the depo- 

 sition of the eggs, and last about twenty four hours. This phenomenon is subjected 

 to a mathematical law as far as it afl:ects the substance, but as there is already present 

 a vital agent, the living principle of the forthcoming being, an allowance is made for 

 variations. Let us consider the subject under both points of view. 



Generally the primitive yolk's sphere divides into two spheres of equal size (figs. 

 23, 24 and 25), then these two are subdivided, making four spheres (figs. 34 and 35), 

 then eight spheres appear (figs. 38 and 39), then sixteen (fig. 43), and so on until the 

 yolk's mass is so much subdivided that it presents the appearance of a mulberry, the 

 mulberry shape or last stage of the division of the vitellus. Such is the physical or 

 mathematical law which governs the matter, as it will strike every one when observing 

 the phenomenon of the division. 



Now the variations. While the foregoing is taking place tiiere are eggs in which 

 the process of the division appears rather fanciful, as if the living principle was already 

 assuming a marked preponderance over the matter itself, and subduing the latter to the 

 power of organism. Thus in fig. 15 one point of the sphere is depressed : in fig. 16 

 the depression has become a groove. The sphere sometimes is split open as in fig. 

 17, Sometimes, again, a groove on one side and a slight depression on the other side 

 are observed (fig. 18). Still others show two symmetrical grooves opposite each other 

 and connected by a transverse line (figs. 19 and 20), which foreshadows a division 



