114 WALTER HEAPE. 



two sides meet to form a hood, covering the hinder part of 

 the area, but anteriorly I have been unable to determine the 

 extent of their growth. 



In the surface view of an embryonic area measuring '97 

 by 79 mm. in diameter, a band of a lighter shade than the 

 remainder is to be seen in the front part of the long axis of 

 the area, its posterior end adjoining the anterior end of the 

 primitive streak; the latter occupies the hinder third of the 

 area, and where it joins the light- coloured band a pit, the 

 upper opening of the neurenteric canal, is distinctly to be 

 seen surrounded by a dark rim. 



In transverse section the light-coloured band is seen to be 

 caused by a diminution in thickness of the epiblast plate 

 and of the mesoblast in the middle line. The epiblast of 

 this region is bent inwards to form a groove, the medullary 

 groove ; it is wide and shallow throughout, and the cells 

 forming it are not more than two rows deep, while the 

 remainder of the epiblast plate, except at the extreme edge, 

 is three cells deep. 



Anterior to the medullary groove a continuous layer hardly 

 differentiated into mesoblast and hypoblast underlies the 

 epiblast plate. In the region of the medullary groove an 

 axial strip of the cells underlying the epiblast exhibits no 

 division into hypoblast and mesoblast ; this portion, though 

 partially separated from the lateral masses of mesoblast, is 

 still connected with them, however, on each side by a narrow 

 neck of cells, and is also directly continuous laterally with 

 the hypoblast. The lateral hypoblast is quite distinct from 

 the superjacent lateral masses of mesoblast. The axial strip 

 of cells underlying the medullary groove (thus shown to be 

 continuous with both the mesoblast and the hypoblast) may 

 be regarded as the commencing notochord. 



The neurenteric canal does not any longer perforate the 

 blastoderm, its upper part alone remaining, which is sur- 

 rounded by a thick mass of mesoblast, causing the dark rim 

 seen in the surface view round the pit. Its anterior wall is 

 connected with the axial mass of cells underlying the medul- 

 lary groove, while its hind wall forms the front end of the 

 primitive streak. 



The surface view of a somewhat older specimen, 1*5 mm. 

 by "81 mm. diameter, shows the medullary groove relatively 

 much longer and more clearly defined. Anteriorly it reaches 

 nearly to the edge of the embryonic area. 



In section it is seen to be shallow at each end, but is much 

 deeper and narrower towards the middle of the embryonic 

 area. Its walls, at the anterior end, are but slightly less 



