DEVELOPMENT OF OTHER VERTEBRATES 



421 



lay eggs with a large amount of yolk and encased in tough egg cap- 

 sules in which development to the juvenile stage occurs. In sharks 

 the large yolk-laden eggs are retained in the o\'iduct of the parent, 

 where they develop, and the young are " born " in the juvenile 

 stage, in which they are known as " pups." Such a mode of 

 development is called ovoviviparous in contrast with the viviparous 



kob 



dj 



oc -- 



dt 



nl a 



.,.>*^ 



Fig. 219. — Embryonic and larval stages in fishes. Above, embrj'oofan 

 elasmobranch, the torpedo, at a stage when the gill slits have reached a con- 

 dition similar to that seen only in the embryos of higher vertebrates (c/. 

 Fig. 287, p. 531). Below, larva of a lung fish, Lepulosiren paradoxa, at a 

 stage when the external gills and general outline have a marked resemblance 

 to the early tadpole stage of an amphibian hke the frog. 



a, position of proctodaeal invagination that forms the anus; df, rudiment of dorsal fin; 

 dt, yolk stalk by which embryo is attached to the yolk; h, heart; k, anterior gill slit; md, 

 posterior end of neural tube; n, nasal pit; nt, blastoderm (c/. Fig. 224 E)\ oh, ear; oc, eye; 

 sch, tail; v, protuberance due to developing kidney (pronephros). (After Ziegler.) 



development in the Mammaha. Many fishes, including the stickle- 

 back, build nests in which the eggs are laid; and in some species 

 the eggs and young are cared for by the female or by the male, 

 or by both parents, as in some species of catfish. Other fishes 

 carry the eggs in a brood pouch or attached to some part of the 

 body. There are thus a great variety of ways in which the young 



