282 MR. G. BBOOK ON THE 



mid and hind brain shows itself before the optic lobes are seg- 

 mented off, and is well marked at the stage of four or five proto- 

 vertebrse. The fore brain seems to be marked ofi" from the 

 posterior portion first, then the posterior portion is afterwards 

 divided into mid and hind brain. About the time that the optic 

 lobes are fully segmented oS, a lumen appears in the mid brain 

 and almost simultaneously one in the fore brain also, while that 

 in the hind brain develops more slowly, not appearing till 8 proto- 

 vertebrse are formed, or even later. 



The exact time at whicb the notochord appears was not noted, 

 but it was already a well-marked feature by the time the optic 

 lobes were outwardly marked out from the keel, when it is to be 

 seen extending well up to the eye-lobes. Posteriorly it widens 

 out, and its cells seem to merge insensibly with those of tbe sur- 

 rounding tissue ; at least, I have not been able to trace it quite 

 up to the edge of the blastoderm. 



Kingsley and Conn state that the notochord originates in the 

 hypoblast, that it is then pushed up through the mesoblast, divi- 

 ding the latter into two lateral plates. I have not been able to 

 verify this statement up to the present. "When first observed, 

 the notochord appeared rather flattened in transverse section, 

 and the lateral mesoblastic plates quite distinct, 



"With the separation of the optic lobes, the two tracts of meso- 

 blast begin to be divided into somites. The first traces of proto- 

 vertebrse were observed after about sixty hours' development, and 

 after the formation of KupfEer's vesicle. 



The thickening of the epiblast forming an invagination for the 

 eye-lens has been observed as early as in embryos with three 

 protovertebrse, but more generally when the embryo possesses 

 about six protovertebrse. My observations of its further de- 

 velopment and the separation of the lens were but a confirma- 

 tion of what is already known on the subject. 



It was difficult to make out the auditory vesicles during the 

 process of invagination, and one only began to recognize them 

 clearly by the time the process was completed. Invagination 

 seems to begin, however, very soon after that for the optic 

 lenses, and they are fully marked ofi", as shown in fig. 19, by about 

 the stage of eight protovertebrse. The lenticular body has an 

 amber tint. About the time that the eye-lenses firsb show traces 

 of invagination, a broadening out of the tissue on each side of the 

 hind brain is seen to begin ; the embryonic border (Embryonal- 



