77? 
XV EGGS 409 
egos 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, < 
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 
Fic. 234.—Different types of egg-segmentation in Fishes. A, a typical telolecithal egg. 
Holoblastic and unequal segmentation in Ama (B) and in Lepidosteus (C). D, the 
meroblastic segmentation of a Teleost. «jp, Animal pole; em, egg-membrane ; 
ma, macromeres ; mi, micromeres ; 7, nucleus; 0.g, oil globule ; p, protoplasm ; 
v.p, vegetative pole; y, yolk. (From Ziegler: A, after Hertwig ; B, after Whit- 
man and Kycleshymer ; °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. 8 2). 
