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 numbers are eaten as larvae 

 by enemies. Between these extremes again there are numerous habit 

 intergrades. Some teleosts such as Gambusia and Anableps, with 

 comparatively few large eggs, practice pairing and intromission of 

 sperm, with resultant viviparity. Members of the same family such 



FIG. 97. Development of Neoceratodus forsteri. A, lens-shaped blastula; 

 B, stage with semicircular blastopore (bl. p,) ; C, later stage in which the blastopore 

 (bl. p.) has taken the form of a ring-like groove enclosing the yolk-plug (ylk. pi.)', 

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

 the rudiment of the medullary folds (wed.); E, stage in which the medullary 

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

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

 mesonephros. (From Parker and Haswell, after Semon.) 



as Fundulus and Cyprinodon, that have smaller and more numerous 

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

 and anal fins during the process of egg and sperm 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 inseminates the eggs. The male pickerel 

 follows the female closely and fertilizes the eggs as soon as laid. Many 

 grades and modifications of this habit occur that gradually lead up 



