394 MARGARET ROBINSON. 



small angular lumps some of which staiu more deeply thau 

 the others. All through the developmeut, until the yolk is 

 completely liquefied, this irregular staining can be noticed. 

 Even at this very early stage some cells, which, as will subse- 

 quently appear, may be regarded as vitellophags, are budded 

 off from the blastoderm into the yolk (fig. 2^ v^.). 



Stage B. 



External View. — The external appearance of the embryo 

 closely resembles that of the next stage (Stage B'). On the 

 ventral surface the blastoderm shows three areas of thicken- 

 ing which are arranged so as to form the apex and two 

 adjoining sides of a triangle. This triangle, however, is much 

 shorter than that outlined by tlie ventral thickenings in 

 Stage B'. It is, in fact, an area which barely covers the 

 posterior two thirds of the ventral surface of the blastoderm. 

 It is outlined by two thickened strands which converge, with 

 a thickened patch uniting their convergent ends. 



These lateral thickened strands consist of large, rounded, 

 granular cells which are thicker and rounder than the cells 

 of the rest of the blastoderm. In some places these cells 

 appear to give rise to others by a kind of tangential division 

 (figs. 3 and 4). That some of the new cells formed in this 

 way are vitellophags there can be little doubt, others are pro- 

 bably some of the first cells of the ordinary mesoderm. 



The median posterior thickening (the thickened apex of 

 the triangle) has in its centre a very narrow groove running 

 in a longitudinal direction. This groove is short, extending 

 through eight sections (each only 4ju in thickness). 



Of these sections I have drawn two, one (fig. G) through 

 the middle region of the groove, and the other (fig. 5) near 

 its anterior limit. 



Behind the region shown in fig. G the groove widens out 

 a little. A section taken immediately behind the groove is 

 shown in fig. 7. 



The section through the middle region shows an inpushiug 



