318 J. S. BUDGETT. 



1 — 6). The blastopore is formed by a more general over- 

 growth of epiblast, and is from the first circular; it is only just 

 before closure that it is possible to tell from an external view 

 which is the anterior and which the posterior side of the pore 

 (figs. 6, 8). It is quite during the last stages of gastrulation 

 that the closing mouth of the gastrula swings up to the 

 posterior edge of the egg. When the blastopore has reached 

 this position it becomes pointed at the anterior end, and there 

 can now be seen running forward from this point a groove 

 showing unmistakably the line of fusion of the edges of the 

 mouth of the gastrula (fig. 9). 



Finally the yolk retreats, and a slit-like open blastopore 

 remains at the posterior end of the pear-shaped neural plate 

 (fig. 10). 



While this fusion has been taking place the centre of 

 gravity has been continually shifting; for along the line of 

 fusion there is a greater accumulation of yolkless protoplasm, 

 i. e. epiblast and mesoblast, than elsewhere. Yolk is heavy, 

 therefore the fused edges of the mouth of the gastrula come to 

 the upper side. 



Finally the entire egg has rotated through 180°. The 

 anterior end of the archenteron is now in the position that the 

 posterior end occupied at the beginning of gastrulation. The 

 area occupied by the neural plate has been formed chiefly by 

 the downgrowth of the lateral and anterior edges of the 

 blastoporic rim ; however, the posterior edge here takes a 

 greater share in gastrulation than in Rana, and in consequence 

 the blastopore comes to lie not at the extreme posterior end of 

 the archenteron, but further towards the middle, while the 

 neural plate extends beyond the blastopore. The anus, how- 

 ever, makes its appearance at the extreme posterior end of the 

 archenteron, far from the position now occupied by the blasto- 

 pore (Sections II, III, an.). 



The centre of the neural plate becomes slightly depressed, 

 and here the blastoporic scar is seen running forwards from 

 the edge of the blastopore along the whole plate as the 

 " primitive streak " (fig. 11). 



