3^ 



COMPARATIVE ANATOMY 



growth period, the result of which may be a very greatly increased body of 

 cytoplasm (Fig. 38). The volume of the extra-nuclear part of the egg is 

 further more or less increased by the acquisition of special food materials, 

 derived from ovarian sources, which are deposited in the cytoplasm in the 

 form of characteristic coarse granules or globules known collectively 

 as the deutoplasm or yolk of the egg. 



Another point of contrast between the ovum and spermatozoon lies 

 in the fact that the latter, consistently with its motile nature, remains a 

 naked cell while the ovum may become invested by membranes or enve- 

 lopes of various sorts, either protective or nutritive. An egg that possesses 

 a bulky yolk-laden cytoplasm is covered by a delicate but tough vitelline 



Fig. 29. — Spermatozoa of dogfish (Squalus), frog, (Rana), parrot (Psittacus), mouse 

 (Mus) and ape (Innus). H, head; M, middle piece; T, tail. The spermatozoon of the 

 frog is about o.i mm. long. (Redrawn from Retzius.) 



membrane (Fig. 38). The bird's egg (Fig. 32) is protected by a hard 

 calcareous shell underlaid by a tough fibrous shell-membrane. Nutritive 

 substance also, such as the albumen or "white" of the egg of the common 

 fowl, may be deposited about the egg. 



The spermatozoa of the various vertebrates are uniformly minute 

 motile bodies whose differences in size and structure are insignificant when 

 contrasted with the differences exhibited by various ova. Eggs differ 

 most remarkably as to the amount of yolk carried by the cytoplasm and 

 as to their outer investments. The microscopic egg of a mammal and the 

 gigantic ostrich egg encased in its hard shell would hardly be suspected 

 of being essentially similar objects. 



Eggs of Fishes 



Even within the group of fishes the eggs differ widely. In the great 

 majority of fishes the eggs are relatively small (less than 5 mm. in diam- 

 eter), contain a correspondingly small amount of yolk, and are invested 



