234 



Basic Structure of Vertebrates 



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

 hardly 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. 200). The following data illustrate the differences in ova in regard 

 to size and content of \ oik : 



Egg 



Amphioxus 



Some frogs 



Domestic fowl (ovum or "yolk") 



Approximate 

 Diameter, mm. 



0.1 



2.0 



30.0 



Relative 

 \ r olumes 



1 



8,000 



•,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. 

 The ovum of the oviparous hummingbird is tremendously larger than 

 that of the viviparous whale. But correlation with size of body may 

 appear when the developmental methods of the animals are similar — 

 e.g., in reptiles and birds. 



The number of eggs produced at a time or in a breeding season 

 ranges from one to several millions. The maximum number occurs in 

 some of the bony fishes — e.g., the cod. Among amphibians, some frogs 

 produce a single egg per season, others 30,000 or more. In reptiles, 

 birds, and mammals, the number is relatively small. 



The eggs of fishes are usually relatively small, less than 5 mm. in 

 diameter. Eggs of sharks and skates, however, contain much yolk and 

 rival in size the eggs of birds. These large eggs are enclosed in shells 

 consisting of a hornlike material secreted by the anterior part of the 

 oviduct. In oviparous sharks and skates, the shell is usually flat and 

 quadrangular and has long tendrils which serve to anchor it to seaweed 

 or other objects (Fig. 198). 



The eggs of amphibians, which always contain considerable yolk, 

 are larger than the eggs of many fishes but smaller than the average 

 for reptiles and birds. Eggs of various frogs range from 1.5 to 3 mm. in 

 diameter. Eggs of large salamanders (Necturus, Cryptobranchas) are 

 5 or 6 mm. in diameter. The amphibian oviduct deposits upon the 

 egg a layer of gelatinous substance which, after the egg has been ex- 

 truded into the water, swells to form a thick, jelly-like envelope 

 (Fig. 199). 



