176 ALICE JOHNSON. 



open part of the blastopore to the anterior pit seems to prove 

 that the blastopore as a whole is dorsal and not ventral. 



A slight additional argument in favour of this view may 

 perhaps be found in the much greater nearness of the archen- 

 teron to the dorsal surface than to the ventral in early stages 

 of development before the yolk has been absorbed. It seems 

 natural that the cavity should exist near the surface from 

 which the involution to form it originally sprung. 



It has already been mentioned that, in the Lizard, the primi- 

 tive streak extends in front of the anus on the ventral surface 

 as far as the opening of the allantois into the alimentary canal. 

 Scott and Osborn described, at a comparatively late stage (with 

 rudiments of external gills, &c.) in the Newt, a very distinct 

 fusion of the hypoblast and epiblast in the middle ventral line 

 behind the mouth. I have myself observed the fusion which 

 they say is connected with the early formation of the thyroid 

 body. Can this also be part of the primitive streak ? If so, 

 neither the mouth nor anus represent the extreme ends of the 

 blastopore. 



A possible connection between the two methods of formation 

 of the mesoblast in Vertebrates, viz. as outgrowths from the 

 primitive streak or lips of the blastopore, and as outgrowths 

 from the hypoblast, is suggested by the theory of an elongated 

 dorsal blastopore. We may suppose that, at a time when the 

 blastopore was a long narrow open slit, the archenteron was a 

 large cavity opening into it in the median line, and the meso- 

 blast consisted of a pair of pouches opening into it on each 

 side for its whole length. When the blastopore became closed 

 and a separation between the epiblast and hypoblast ensued, 

 the mesoblast naturally retained its connection with the latter^ 

 since it was functionally from the beginning an appendage of 

 the archenteron. Of course, where the primitive streak existed 

 the mesoblast would keep as far as possible traces of its 

 original condition, but in regions where the primitive streak 

 was obliterated the mesoblast could only proceed from the 

 hypoblast. 



In conclusion, I wish to express my very sincere thanks to 



