No. I.] GAR-PIKE AND STURGEON. 43 



37, 38). In PI. Ill, Fig. 51, the head outline is entirely an 

 epiblastic thickening : anteriorly it is a pad of almost uniform 

 thickness ; caudad the thickness of epiblast tapers away, but at 

 the rim of the blastopore suddenly increases, dipping down into 

 the undifferentiated tissue. These characters of the epiblast 

 may be well seen in the (nearly) sagittal section of PI. IV, 

 Fig. 66. 



The central nervous system is earliest developed at its ex- 

 treme ends, brain and neurenteric canal. Between the con- 

 dition figured in PI. Ill, Fig. 51, and in section PI. IV, Fig. 66, 

 and that of the stage of PI. Ill, Fig. 52, and of the sagittal 

 section PI. IV, Fig. 71, — slightly earlier — notable changes 

 have occurred : there has been a concentration of formative 

 epiblast in the brain region ; its anterior end has dipped deeply 

 down, and has acquired a lumen, whether originally by cell dis- 

 association or by process of invagination the writer has not 

 been able to determine. He is certain, however, that in the 

 initiatory process the cell thickening was deep and sharply 

 marked in the median line. In the section given it is of 

 especial interest to note the point of union, x, of the front end of 

 the brain with the formative epiblast : it is situated at a remark- 

 able distance tailward. A cross section at this point is figured 

 in PI. IV, Fig. 72. It is evident from the longitudinal section 

 that the extension of the brain forward has been caused by the 

 increase in size of its sides and floor. The width and flatness 

 of the ventricle should be noted. By study of serial sections 

 tailward of this stage it is found that the neural canal 

 becomes thicker (shallower) and narrower: there occurs no 

 evidence that the central canal has been formed by cell dis- 

 association ; the formative epiblast rounds abruptly into it, 

 and it appears broadly trench-like : it is roofed, however, 

 dorsally by a strip of cells which appear in contact with both 

 Deckschicht and marginal cells of formative epiblast (PI. IV, 



Fig. 73)- 



In the present paper it is not intended to discuss the studies 

 of Kupffer on the morphology of the head of vertebrates. His 

 admirable description of the later development (beginning with 

 the 45th hour) of the brain of Acipenser has been followed by 



