166 



VERTEBRATE ZOOLOGY 



The males and females simply swim about in schools, emitting eggs 

 and sperm. The eggs are fertilized in large numbers and float about 

 near the surface of the sea. Only a small percentage of them complete 

 their development to hatching and large numfiers are eaten as larvae 

 by enemies. Between these extremes again there arc numerous habit 

 intergrades. Some teleosts such as Gambusia and Anablcps, with 

 comparatively few large eggs, practice pairing and intromission of 

 sperm, with resultant viviparity. Members of the same familj' such 



Fig. 97. — Development of Nincemtndns forsWri. A, leas-shaped blastula; 

 B, stage with semicircular blastopore {hi. p,); c, later stage in which the lilastopore 

 {bl. p.) has taken the form of a ring-like groove enclosing the yolk-plug {ylk. pL); 

 D, stage in which the narrow medullary groove (Up. sut.) has appeared with 

 the rudiment of the medullary folds {med.); E, stage in \\hich the medullary 

 folds {med.) have tiecome well developed; F, later stage with well-formed head 

 and two visceral arche.s (vise.) and rudiments of eye (eye) and ear (and.); pron, 

 mesonephros. (From Parker and Ilaswell, after Hemon.) 



as Fundulus and Cyprinodon, that have snKill(>r and more numc^'ous 

 eggs, practice pairing, the males clasping the fenuUcs with their dorsal 

 and anal fins during the process of egg anti s]xn-m emission. Thus there 

 is less chance of a failure of insemination. The female^ of some fishes 

 such as the black bass, sticklebacks, etc., lay eggs in a nest and the male 

 follows her into the nest and ins(>niinates the eggs. The male pickerel 

 follows the female closely and fer1 ilizes the eggs as soon as laid. ]\taiiy 

 grades and modifications of (his habit occur (liat gi-adually lead up 



