96 



THE GERM-CELLS 



Nereis (Fig. 43). Often associated with the proper deutoplasm- 

 spheres are drops of oil, either scattered through the yolk (Fig. 43) 

 or united to form a single large drop, as in many- pelagic fish-eggs. 

 The deutoplasm is as a rule heavier than the protoplasm ; and in 



such cases, if the yolk is accumulated 

 in one hemisphere, the egg assumes a 

 constant position with respect to gravity, 

 the egg-axis standing vertically with the 

 animal pole turned upward, as in the 

 frog, the bird, and many other cases. 

 There are, however, many 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 constitu- 

 ents they usually cause the Q.gg to lie 

 with the animal pole turned downwards, 

 as is the case with some annelids 

 {Nereis) and many pelagic fish-eggs. 



3. The Egg-envelopes 



en 



Fig. 44. — Schematic figure of a 

 median longitudinal section of the 

 egg of a fly (Musca), showing axes 

 of the bilateral egg, and the mem- 

 branes. [From KORSCHEI.T and 

 Heider, after Henkino and Bloch- 



MANN.] 



e.n. The germ-nuclei imiting; ;;/., 

 micropyle ; p.h. the polar bodies. 

 Tlie 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 peripheral 

 or peri-vitelline layer of protoplasm. 

 The outer heavy line is the chorion, 

 the inner lighter line the vitelline 

 membrane, both being perforated by 

 the micropyle, from which exudes a 

 mass of jelly-like substance. 



The egg-envelopes fall under three 



categories. These are: — 

 {a) The vitelline membrane, secreted 



by the ovum itself. 

 {b) The ehorion, formed outside the 

 ovum by the activity of the 

 maternal follicle-cells, 

 (t) Aeeessory 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 Qgg 

 is naked but forms a vitelline mem- 

 brane almost instantaneously after the 



