98 ARTHUR ROBINSON, M.D., AND RICHARD ASSHETON, M.A. 



lateral lips will be cut through in forty sections, it requires close upon 

 eighty sections to cut through the fused mass at the ventral lip. 



In fig. 10 the levels of the sections (figs. 7, 8, 9) are indicated by 

 the lines 7, 8, 9. 



Thus a sagittal section through the blastopore ought to show a 

 very much larger mass of fused layers at its ventral lip than at its 

 dorsal. 



This is seen to be the case as shown in fig. 5, which is a sagittal 

 section through the blastopore of a rather older frog embryo. The 

 blastopore has closed to a considerably greater extent than was the 

 case in the embryo we have just been describing. The actual extent 

 of the closure of the blastopore by the concrescence of the latero- 

 ventral lips is represented, we think, by the distance between the 

 point x and the present edge of the ventral lip of the blastopore. 



We find by a series of measurements that at whatever stage during 

 the closure of the blastopore, the section be taken, the distance 

 between point x and the edge of the dorsal lip of the blastopore is 

 always approximately the same, and the same as the diameter of the 

 blastopore at its first commencement. We say approximately, for, 

 owing probably to variation in size of the egg, the measurements do 

 not exactly agree. 



Fig. 11 was drawn from a living specimen in which the blastopore 

 had become greatly reduced, and in which the neural plate and 

 neural groove were distinctly visible along the dorsal surface. 



From the lower, or ventral, lip of the open blastopore stretches 

 a very faint (too deeply drawn in the figure) line, which probably is 

 the remaining trace of the line of concrescence of latero-ventral blasto- 

 pore lips. 



Fig. 13 is a section taken at right angles to this line, not of the 

 same embryo, but of one about the same age. In this the line is 

 seen to be a groove (PG) on the surface, below which all these 

 layers are fused. 



We will at once call this groove primitive groove, and the fused 

 mass below it primitive streak ; for, as we hope to prove in the sequel, 

 there can be no reasonable doubt that these structures are the 

 homologue of those parts in the chick embryo to which these names 

 were originally given. 



