THE EGG 



29 



where the egg joins the germinal epithelium, so that when the egg severs itself an 

 opening, the micropyle, is left. 



The marine planaria offer a further example of a simple egg- formation (Fig. 24, D}. 

 These possess very numerous ovaries, whose nourishment is provided for by their 



kb ksch 



FIG. 24. A, -B, C, Three ovarian eggs of a mussel in different stages of development, after 

 Fleming, m, Micropyle ; dh, yolk membrane. D, Ovary of a marine planarian with eggs in 

 different stages of development ; da, branch of the intestine or gastro-canal ; U, germinal layer ; 

 e, advanced egg. 



position between the branches of the intestine and close to its walls. In each ovary 

 there is a small germinal layer (kl) formed by a mass of small cells with nuclei and 

 little protoplasm. Some of these cells grow and 

 become eggs, numerous yolk granules forming in 

 them, and their nuclei changing into the charac- 

 teristic germinal vesicle. Other cells remain 

 small ; they take up a position between the 

 strongly growing egg cells, and so form a frame- 

 work in the ovary, which is continued into the 

 oviduct. 



As a type of a perfect meroblastic telolecithal 

 egg with complicated envelope-formation we have 

 the bird's egg (Figs. 25 and 26). The egg is 

 fertilised within the mother body, and has 

 already begun to develop when it is laid. The 

 different parts which can then be recognised have 

 very different meanings and origins. In the 

 interior of the egg we recognise the yellow yolk 

 sphere, the well-known yellow of the egg (Fig. 25). This is formed in the ovary, 

 and represents the real egg. In the ovary it is merely a simple meroblastic egg cell, 

 consisting of the following parts : 



FIG. 25. Ovarian egg cell of a Fowl. 

 ksch, Formative yolk ; kb, germinal 

 vesicle ; wd, white yolk ; gd, yellow yolk ; 

 dh, yolk membrane. (After O. Hertwig.) 



