CHORDATE ANATOMY 



'- EGG-CASE 



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 horn-like 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. 26) 

 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, Cryptobranchus) 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 

 extruded into the water, swells to form a 

 thick jelly-like envelope. (Fig. 27) 



Reptiles and birds produce eggs con- 

 taining an enormous amount of yolk (Figs. 

 28, 36). The protoplasm in these great 

 eggs is aggregated at one spot on the surface 

 of the egg, marking the animal pole, while 

 the remainder of the egg is yolk nearly, if not 

 quite, devoid of protoplasm. The local- 

 ized protoplasm (germ-disc; Fig. 36) 

 appears as a small white fleck on the sur- 

 face of the yellow yolk. Before the egg is 

 fertilized the germ-disc contains a single 

 nucleus. These large eggs are invested by 

 a tough vitelline membrane external to 

 which may be more or less nutritive 

 albumen (the "white" of a hen's egg) 

 and an outer shell which in most reptiles 

 is of a leathery texture, but in crocodiles, 

 alligators and birds is highly calcified and therefore hard and brittle. 



Eggs of mammals, with two exceptions, are minute, containing a 

 minimum of yolk. The exceptions are the duck-bill (Ornithorhynchus) 

 and the spiny ant-eater (Echidna) of the Australian region. These 

 two mammals, presumably of primitive type, lay large eggs encased in 

 tough shells. In general these mammals are reptilian in their methods 



Fig. 26. — Egg-case of small 

 shark. T, tendrils coiled around 

 branches of a horny (gorgonian) 

 coral. About half actual size. 

 (Drawn from specimen in the 

 anatomical collection of the 

 Biological Laboratories, Harvard 

 University.) 



