REPRODUCTION 



39 



by only thin and soft protective membranes. The eggs of sharks and 



skates, which contain enormous yolk masses, rival in size the eggs of 



birds, and are enclosed in cases or shells which are secreted by the wall 



of the anterior portion of the oviduct. These shells consist of a substance 



resembling chitin or horn. In viviparous 



sharks and skates the shell is very thin and 



soft. In those which are oviparous the 



case (Fig. 30) becomes thick and stiff and 



usually acquires a curious flattened 



elongated quadrangular form, often with 



long curling tendrils extending from each 



corner. These tendrils serve to anchor the 



egg by becoming entangled with seaweed 



or other objects. In some sharks the 



egg-cases are of even more compHcated 



form. 



Eggs of Amphibians 



The eggs of amphibians, which always 

 contain a considerable amount of yolk, are 

 larger than the eggs of many fishes, but 

 smaller than the average for reptiles and 

 birds. Eggs of various species of frog range 

 in diameter from 1.5 to 3 mm. Eggs of 

 the large salamanders, Necturus and 

 Cryptobranchus (the hellbender), range be- 

 tween five and six millimeters in diameter. 

 The amphibian oviduct deposits upon the 

 egg a layer of gelatinous substance (Fig. 31) 

 which, after the egg has been extruded into 

 the water, swells to form a thick jelly-like 

 envelope. In some amphibians (e.g., 

 Cryptobranchus, most toads) toughened 

 strands of the jelly pass from egg to egg, 

 thus tying many eggs together to form a 

 long string (Fig. 31C). 



• )^ EGG-CASE 



Fig. 30. — Egg-case of small 

 shark. T, tendrils coiled around 

 branches of a homy (gorgonian) 

 coral. About half actual size. 

 (Drawn from specimen in the 

 anatomi cal collection of the 

 Biological Laboratories. Harvard 

 University.) 



Eggs of Reptiles and Birds 



In accord with other evidences of the close relationship of reptiles 

 and birds, the eggs, as well as the methods of development, of the 

 animals of these two groups are closely similar. The eggs are large 

 both absolutely and in proportion to the size of the animal. There is 



