DEVELOPMENT OF TRACHTNUS YIPERA. 289 



guishable, but afterwards this comes in close contact with the 

 zona radiata, and. of ten requires the action of reagents to show it 

 properly. 



My observations appear to confirm those of Henneguy, that 

 the invagination observed in optic section in the living egg is an 

 inward folding of the lower layer cells of the epiblast, and that 

 afterwards the alimentary tract is built up from this layer, to- 

 gether with materia] derived from the intermediary layer. This 

 point cannot, however, be settled definitely without a careful 

 examination of sections of this stage. 



My observations confirm those of Ryder as to the nature 

 and persistence of the segmentation-cavity, and in this respect 

 pelagic teleostean eggs seem to differ from all others hitherto 

 described. 



Although the heart a})pears early on the fourth day, its venous 

 end remains closely applied to the vitellus up to several days after 

 hatching, and I have not been able to find any vascular system 

 either in the embryo or in the vitellus up to 14 or 15 days after 

 development begins^ that is 4 or 5 days after hatching. In this 

 respect the observations of Ryder and Kingsley and Conn agree 

 with my own, although in non-pelagic teleostean eggs an elaborate 

 circulatory system is developed both in the vitellus and in the 

 embryo a considerable time before hatching. 



I have nothing new to record in the later stages of develop- 

 ment. The liver and the pancreas arise as little pouches budded 

 off from the mesenteron ; the proctodeum arises late, but is well 

 formed at the time of hatching. The stomodeum does not appear 

 to develop until the embryo has left the shell. The young fish 

 usually hatch out on the tenth or eleventh day after impreg- 

 nation, the early ones with little pigment on the eyes and body, 

 the later ones with the pigment much more developed. 



DESCRIPTION OF THE PLATES. 



Lettering used throughout Plates. 



b. c. = breathing-chamber. 



h. — hypoblast. 



b. = blastoderm. 

 s. c. = segmentation cavity. 

 b. r. = blastodermic rim. 



It. = keel. 



0. I. — optic lobes. 



a. s. = auditory sacs. 



k. V. = Kupffer's vesicle. 



b. p. = blastopore. 

 h. — heart. 



e. s. = embryonic shield. 



