REPRODIKTION 



27 



the yolk or deutoplasm. The egg may become invested by membranes 

 or envelopes, either protective {e.g., the vitelline or yolk membrane; 

 the hard calcareous shell of a bird's egg; see Fig. 28) or nutritive (e.g., 

 the albumen or "white" of a bird's egg). 



Eggs differ most remarkably as to the amount of contained yolk 

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

 the gigantic ostrich egg encased in its hard shell would seem to be hardly 



Fig. 25. — Spermatozoa of dogfish (Squalus), frog (Rana), parrot (Psittacus), mouse 

 (Mus) and ape (Inuus). H, head; M, middle piece; 7", tail. The spermatozoon of the 

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



comparable objects. The thing referred to in kitchen and market as an 

 "egg" consists of the egg in strict sense, or ovum, plus various extraneous 

 substances and structures. The hen's ovum, corresponding to the small 

 egg of some fish, is merely the yellow sphere commonly called the "yolk" 

 of the "egg," enclosed in its vitelline membrane (Fig. 28). The following 

 data illustrate the differences in eggs in regard to size and content of yolk: 



Egg 



Amphioxus 



Some frogs 



Domestic fowl (ovum or "yolk' 



Approximate 

 diameter, mm. 



o. I 



2.0 



30.0 



Relative 

 volumes 



8,000 

 2 7 , 000 , 000 



The volume of an ostrich ovum would be hundreds of millions of times 

 greater than that of a mouse egg whose diameter is about 0.06 mm. 

 Size of eggs is correlated primarily with the method of development. 

 Correlation with size of body may appear when the developmental methods 

 of the animals are similar, e.g., in reptiles and birds. 



