132 



THE GERM-CELLS 



cases in which the egg may lie in any position. When fat-drops are 

 present they usually lie in the vegetative hemisphere, and since they 



are lighter than the other constituents 

 they usually cause the egg to lie with 

 the animal pole turned downwards, as 

 is the case with some annelids (Nereis} 

 and many pelagic fish-eggs. 



en 



Fig. 62. Schematic figure of a 

 median longitudinal section of the egg 

 of a fly (Afusca), showing axes of the 

 bilateral egg and the membranes. 

 [From KORSCHELT and HEIDER, 

 after HENKING and BLOCHMANN.J 



e.n. The germ-nuclei uniting; 

 m. micropyle; p.b. the polar bodies. 

 The flat side of the egg is the dorsal, 

 the convex side the ventral, and the 

 micropyle is at the anterior end. 

 The deutoplasm (small circles) lies 

 in the centre surrounded by a periph- 

 eral or peri-vitelline layer of proto- 

 plasm. The outer heavy line is the 

 chorion, the inner lighter line the 

 vitelline membrane, both being per- 

 forated by the micropyle, from which 

 exudes amass of jelly-like substance. 



3. TJie Egg-envelopes 



The egg-envelopes fall under three 

 categories. These are : 



(a) The vitelline membrane, secreted 



by the ovum itself. 



(b) The chorion, formed outside the 



ovum by the activity of the 

 maternal follicle-cells. 



(c) Accessory envelopes, secreted by 



the walls of the oviduct or other 

 maternal structures after the 

 ovum has left the ovary. 

 Only the first of these properly be- 

 longs to the ovum, the second and third 

 being purely maternal products. There 

 are some eggs, such as those of certain 

 ccelenterates (e.g. Renilla), that are 

 naked throughout their whole develop- 

 ment. In many others, of which the 

 sea-urchin is a type, the fresh-laid egg is 

 naked but forms a vitelline membrane 

 almost instantaneously after the sperma- 

 tozoon touches it. 1 In other forms (in- 

 sects, birds) the vitelline membrane may 

 be present before fertilization, and in 

 such cases the egg is often surrounded 

 by a chorion as well. The latter is. 

 usually very thick and firm and may 

 have a shell-like consistency, its surface 

 sometimes showing various peculiar 

 markings, prominences, or sculptured 

 patterns characteristic of the species 

 (insects). 2 



1 That the vitelline membrane does not preexist seems to be established by the fact that 

 egg-fragments likewise surround themselves with a membrane when fertilized. [HERTWIG.] 



2 In some cases, according to Wheeler, the insect-egg has only a chorion, the vitelline 

 membrane being absent. 



