FORMATION AND FATE OF THE PRIMITIVE STREAK. 469 



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 

 thinkj 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 pro- 

 bably is the remaining trace of the line of concrescence of 

 latero-ventral blastoporic 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 



