Reproduction 



239 



BROOD- POUCH 



smaller fish eggs, scantily endowed with yolk, develop rapidly and 

 soon become free-living and self-supporting while still very minute. 

 The miniature fish then enters upon a long period concerned mainly 

 with feeding and growth. Eggs containing larger quantities of yolk 

 and, in some cases (e.g., sharks and skates), invested by a protective 

 shell pass through a longer period of develop- 

 ment, and the young fish attains relatively 

 large size before it is obliged to obtain food 

 from an external source. The embryo and 

 young of the viviparous fish not only receive 

 maximum protection but may obtain from 

 the mother some food in addition to the initial 

 supply of yolk. In so-called " placental" 

 sharks, the wall of the oviduct develops 

 highly vascular folds or processes, and similar 

 folds arise on the abdominal wall of the em- 

 bryo. The two sets of projecting structures, 

 maternal and embryonic, become closely 

 approximated, thus providing for diffusion of 

 substances from the blood of one to that of 

 the other. 



Among amphibians there is, in general, 

 better provision for protection of eggs and 

 young than in fishes. The great majority of 

 them lay the eggs in water — always fresh 

 water except in the case of a large toad 



(Bufo marinus) of the American tropics. Nests and guarding of 

 eggs are common. Among frogs and toads occur various peculiar ways 

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

 toad carries the long strings of eggs wound about his body and legs 

 until the tadpoles emerge. In some cases eggs are carried in the mouth 

 or vocal pouch of the male. In the South American "marsupial" frog, 

 the eggs develop in a capacious pouch formed in the skin on the back 

 of the female. The eggs of the toad Pipa americana develop in indi- 

 vidual vesicles in the skin on the back of the mother. Viviparity, 

 affording a maximum of protection, occurs in a few amphibians, in- 

 cluding representatives of each of the three orders, Urodela, Anura, 

 and Apoda (Gymnophiona). 



The amphibian egg, whether laid in the open or enclosed in some 

 protective way, develops rapidly into a highly characteristic larva, 

 the tadpole or "polliwog" (Figs. 203, 343), which, with its functional 

 gills and locomotor tail, as well as in many features of internal anatomy, 

 is a distinctly fishlike animal and. if its environment is external water, 



Fig. 202. Sea horse 

 (Hippocampus): male, 

 with brood-pouch. (After 

 Boulenger. Courtesy, 

 Neal and Rand: "Chor- 

 date Anatomy," Phila- 

 delphia, The Blakiston 

 Company.) 



