Development o/" Gastrosteus spinachia. 493 



The course of development is very slow as compared with 

 pelagic ova. It was not until the fourth daj that nuclei 

 became visible in the periblast, and the corneous layer differ- 

 entiated from the " lower layer " cells. On the sixth day the 

 marginal rim is defined and the embryonic scutum indicated, 

 the embryonic thickening being also apparent about noon on 

 that day, by which time the blastoderm invests barely one 

 third of the yelk. By the eighth day two thirds of the yelk- 

 surface are enveloped, and the blastoderm is somewhat de- 

 pressed. The portion of the yelk-surface not yet invested is 

 dotted (with some approach to regularity) with nuclei. 

 Round each nucleus, which is multinucleolate, protoplasm 

 gathers and sends out radiating pseudopodia. Large cells 

 also occur and refringent particles are abundant. Meanwhile 

 the cephalic portion of the embryo is increasing in thickness, 

 so that the keel prominently projects on the ventral blasto- 

 dermic surface ; the optic vesicles are rudely indicated, and 

 the neurochord is differentiated, growing down as the noto- 

 chord appears; and before the close of the eighth day the 

 mesoblastic plates are well defined. The blastoderm, external 

 to the embryo, assumes a striking appearance, as clear vesicles 

 can be discerned scattered numerously over it. These nuclei, 

 possibly periblastic, have a rounded outline and exhibit 

 several nucleoli. Epiboly continues during these changes, 

 and on the twelfth day the closure of the blastopore is effected. 

 Many of the nuclei just noted nov/ approach each other and 

 coalesce. Segmentation of the embryonic trunk proceeds 

 rapidly, and on this day twelve protovertebr^ are marked off. 

 On the following day (the thirteenth) four more are segmented, 

 the primitive optic vesicles are pushed in, and the lenses 

 developed ; the otocysts also appear ; the nasal pits are distin- 

 guishable and the cranial divisions are rudely marked. By 

 the fourteenth day the embryo has appreciably lengthened ; 

 Kupfer's vesicle (y\ hich appears just before the closure of the 

 blastopore, and attains its maximum shortly after) still per- 

 sists, though reduced in size ; the cranial region is greatly 

 advanced, an enteric strand of cells is being aggregated in 

 the mid-ventral region, and nineteen protovertebrse can be 

 counted. A pectoral swelling is visible, indicating the 

 growing heart. This organ rapidly develops, and by the 

 seventeenth day assumes its characteristic campanulate shape. 

 By the dehiscence of the yelk and the splanchnic mesoblast of 

 the embryo a chamber is formed round the heart. At this 

 time the caudal end of the young becomes free, the embryonic 

 tin passing as a median membrane along the dorsum round 

 the termination ot the tail, along the ventral ridge, to the anal 



