REPRODUCTION 49 



filamentous extensions which obviously correspond to the "curling 

 tendrils" (Fig. 30) which serve to anchor the deposited eggs of some other 

 elasmobranchs. It is quite certain, too, that there can be no direct genetic 

 relation between the placenta of the mammal and the placenta-like arrange- 

 ments in some sharks. 



In Amphibians. The circumstances attending reproduction and devel- 

 opment are, if possible, even more diverse among amphibians than among 

 fishes. In point of initial yolk content, however, amphibian eggs are less 

 variable than fish eggs. The smallest amphibian eggs are much larger 

 than the smallest fish eggs and the largest amphibian eggs are small com- 

 pared to the eggs of sharks. 



Viewing the Class Amphibia as a whole and remembering that it is a 

 very small group compared to fishes, it is probably fair to say that amphib- 

 ians show much more concern — in a figurative sense — for the survival of 

 their eggs than do fishes. The great majority of them lay their eggs in 

 water, and it is invariably fresh water except in the case of the large 

 toad (Bufo marinus) of the American tropics. The eggs are commonly 

 deposited in a locality which seems to be instinctively chosen as favorable. 

 They may be attached to the under surface of rocks or other submerged 

 inanimate objects or to the leaves of aquatic plants. Some tree frogs 

 (Hylidae) build a low wall of mud to form a rude hollow nest either at the 

 bottom of shallow water or on shore very near the edge of the water. 

 Jamaican tree frogs place their eggs in the small natural aquaria caused 

 by retention of rain water caught by the leaves of certain large-leaved 

 plants. Some frogs of the eastern hemisphere depart from water so far 

 as to deposit the eggs on leaves or stones or in mud but always near water 

 and the newly hatched young go immediately into the water. 



Many salamanders deposit the eggs on land. They may be placed 

 in small hollows in soft moist earth under stones or logs, but usually very 

 near some body of water. Some salamanders are wholly terrestrial, 

 depositing the eggs in crevices in logs deep in the woods, or on moss and 

 remote from water. The females of some salamanders and of some 

 caecilians (Gymnophiona) protect their eggs by coiling the body around 

 the mass of eggs, the caecilian eggs being deposited in a burrow. 



Certain frogs and toads have various specialized and peculiar ways 

 of caring for the eggs and young. The male of the European "obstetric 

 toad" (Alytes obstetricans) picks up the long strings of eggs extruded by 

 the female and winds them about his body and hind legs. They are 

 carried thus until the tadpoles emerge. The male of the South American 

 frog, Rhinoderma, carries the eggs in his vocal pouch, a capacious recess 

 which communicates with the mouth cavity. 



In several instances the back of the female serves in one way or another 

 as a temporary "nest" for the young. The tadpoles of some anurans are 



