EGGS 



409 



eggs are very small, those of the Plaice, which are exceptionally 

 large, varying from 1-65 to 1'95 mm. 



An egg-cell consists of living protoplasm and a nucleus, a 

 variable quantity of non-living food-yolk, and of certain envelop- 

 ing and protective egg-membranes. The ova of Fishes differ 

 principally in the amount and disposition of the food-yolk, in the 



a.p 



Fig. 234. — Different types of egg-segmentation in Fishes. A, a typical teloleeitlial egg. 

 Holoblastic and unequal segmentation in Amia (B) and in Lepidosteiis (C). D, the 

 meroblastic segmentation of m. Teleost. a.p, Animal pole; e.m, egg - membrane ; 

 ■ma, niacromeres ; mi, micromeres ; n, nucleus ; o.g, oil globule ; p, protoplasm ; 

 v.p, vegetative pole; y, yolk. (From Ziegler: A, after Hertwig ; B, after Whit- 

 man and Eycleshymer ; C, after Eycleshymer.) 



character of the egg-membranes, and in the presence or absence 

 of special perforations in the egg-membranes for the entrance of 

 spermatozoa into the eggs. In the small ova of some of the 

 lower Chordata {e.g. Amphioxus), where the very small quantity 

 of food-yolk is uniformly distributed, and its presence affects all 

 parts of the egg alike, the process of segmentation which follows 

 fertilisation results in the transformation of the entire egg into 

 a mass of approximately equal-sized cells or blastomeres (Fig. 82). 



