VERTEBRATA ACRANIA. 147 



DEVELOPMENT. 



The breeding-season commences about the end of March and 

 lasts throughout the summer. The eggs are laid about an hour 

 after sunset, and are fertilized at once by sperms shed over them 

 by the male. Development begins an hour later and at first goes 

 on very rapidly, so that by sunrise next morning free-swimming 

 flagellated embryos escape from the eggs. Twenty-four hours 

 later the flagellated embryo has become an elongated unsym- 

 metrical larva, with mouth, anus, and one gill-slit. This completes 

 what may be termed the embryonic development. 



The larval development which now succeeds extends over a 

 much longer period. The free-swimming larva is for some time 

 extremely unsymmetrical, but gradually becomes less so. At the 

 same time the adult structure is slowly assumed ; ultimately the 

 free-swimming life is given up, and this closes the larval period. 



I. EMBRYONIC DEVELOPMENT. Cleavage (Segmentation). 

 The ovum, in spite of its small size (^Q of an inch in diameter), 

 contains a considerable number of yolk-granules. It is covered 

 by a delicate vitelline membrane, which becomes separated from 

 it as soon as fertilization has been effected. Only one polar body 

 has been observed, and this rests on the upper pole of the 

 oosperm. Cleavage is complete (holoblastic) and nearly regular. 

 It occupies from three to four hours. The first division takes 

 place in a vertical plane and results in two cells of equal size. 

 These are then equally halved by a second vertical division at 

 right angles to the first. The third division is horizontal (equa- 

 torial) and separates four rather smaller upper cells from four 

 rather larger lower cells in which more numerous yolk-granules 

 are present. These eight cells are now bisected by two vertical 

 divisions making angles of 45 with the earlier vertical ones. The 

 16-celled stage so constituted becomes a 32-celled one by two 

 new equatorial divisions, the planes of which are respectively 

 above and below the first equatorial division. From this point 

 cleavage proceeds more irregularly. It ultimately results in a 

 spherical blastula (blastosphere), which may be compared to a 

 hollow ball with a large central cavity (blastoccele, segmentation 

 cavity) and a wall composed of a single layer of cells, which in 

 the upper half are smaller than in the lower half (Fig. 40). The 



