FOEMATION AND FATE OP THE PEIMITIVE STEBAK. 471 



The lengthening of the embryo has removed from the blas- 

 topore all trace of the '' yolk-plug." 



E-unning ventrally from the blastopore is the primitive 

 groove, a rather narrow but now sharply defined groove near 

 the ventral end, which is suddenly deepened into a pit. 



Beyond the pit the groove continues a short dis- 

 tance. This pit is the commencement of the anus, which will 

 shortly perforate at this spot. In fact, in the model from which 

 this figure was partly drawn the anus had just perforated. In 

 the sections, however, about to be described as representing this 

 stage the anus has not quite perforated. On either side of the 

 primitive groove a distinct ridge is visible. 



The bracket (PS) in the figure includes the whole of the 

 primitive streak from end to end. 



The sections 14, 15, 16, 17, are horizontal sections taken 

 along the lines 14, 15, 16, 17, in fig. 23, and are, therefore, 

 taken at right angles to the longitudinal axis of the primitive 

 streak. 



Fig. 14 is a camera drawing of a section through the blas- 

 topore (line 14, fig. 23), which is nearly closed in. 



In this section the epiblast, mesoblast, and hypoblast are 

 seen to be all fused at the lips of the blastopore. It may be 

 noted that the mesoblast, close to the lip of the blastopore, 

 seems to be divided into two layers: a very dense compact 

 layer {ME') next the epiblast, in which the cells are so closely 

 packed as to render it quite impossible to draw each cell by 

 camera ; while inwardly the mesoblast-cells {ME") are much 

 looser, and are more deeply pigmented and easily traceable by 

 help of the camera. In fact, one is almost inclined to say that 

 the former are mesoblast-cells of epiblastic origin, while the 

 latter are mesoblastic cells of hypoblastic origin. 



Fig. 15 is a section taken about midway between the blas- 

 topore and the anal pit; that is to say, across the middle of the 

 primitive streak. This section is directly comparable to that 

 just described, except that the lips of the blastopore 

 have met and fused, as described in the earlier part of this 

 paper. The surface is distinctly grooved, the edges of the 



