388 ARTHUR T. MASTERMAN. 



blastopore is uudergoiug a process of closure by an approxi- 

 mation of the lips in the median line. The closure proceeds 

 from behind forwards^ and the line of closure is indicated by 

 the faint median groove. 



At the next stage the growth, both laterally and ventrally, 

 of the anterior region has altered the outline of the embryo 

 (figs. 9 and 14). The blastopore has closed for the greater 

 part of its extent, and it is now rather broader from side to 

 side than before. The closure having almost ceased, the 

 blastopore, or rather its anterior end which is still open, 

 broadens out and soon after this becomes circular; it is defi- 

 nitely to be identified as the larval mouth, or more accu- 

 rately the opening into the "stomach." The ventral groove 

 can now be barely distinguished except by careful arrange- 

 ment of the light, and after this stage is quite indistinguish- 

 able. 



The ventral flexure of the anterior part of the embryo is 

 very marked (fig. 9), and in the next stage (figs. 10 and 15) 

 has proceeded so far that the mouth is completely covered over 

 by this part, now to be identified as the "hood" or pre-oral 

 lobe. 



The period included between these two last stages appears 

 to be a fairly long one, during which some important internal 

 differentiations are in progress. 



So far as external observation can show, we may notice 

 that the gastrulation is embolic, the blastopore closes from 

 behind forwards, its anterior part persisting as the "mouth." 

 We may here anticipate a consideration of the work of 

 other observers by saying that in no embryo have I been able 

 to find either in external view or in section any trace of the 

 " posterior pit " of Caldwell (3) as a posterior persistent 

 portion of the blastopore. 



Fig. 25 is a median vertical section through fig. 11. In 

 it the larger granular cells are seen to be invaginating. 

 Throughout the whole series of sections the histological 

 characters of the hypoblast are markedly different from those 

 of the epiblast. Apart from the differences of size aud number, 



