Experiments on the Early Development of the Amphibian Embryo etc. 625 



When first formed tlie medullary folds extend around the blasto- 

 pore on either side nearly to it's posterior end. As they gradually 

 fuse beginning at the anterior end of the embryo, it is evident that 

 when the fusion reaches the blastopore it must stop there unless 

 some such migration takes place. The folds along the sides of the 

 blastopore cannot come together over the yolk plug. 



The fusion does not stop, however, for the folds along the sides 

 of the blastopore disappear. Furthermore the dorsal lip of the 

 blastopore, in the median line only, begins to grow backward 

 over the yolk plug as seen in figs. 6 and 19. Such a growth could 

 only take place by »new tissue coming up to the middle line from 

 the sides, and placing itself with or behind the cells already present 

 in the dorsal lip«. 



Hertwig says that in normal development the blastopore border, 

 with the exception of the small portion which becomes the anus, 

 thus serves for the origin of the dorsal organs, but that the greater 

 portion of the border remained unused in his salt embryos (Rana 

 esculenta), and simply surrounded the yolk fields. 



It is interesting to note that in these Amblystoma embryos 

 whose development was carried so much farther than that of his 

 Ranas, we find a similar condition at first, but later on there is 

 good evidence that the greater portion of the border does take part 

 in the formation of the dorsal organs. 



The fact that this backward growth is compelled by the pre- 

 sence of the yolk plug to take place horizontally instead of verti- 

 cally, and the fact that the posterior lip of the blastopore remains 

 free from yolk cells leaves the opening of the blastopore, the anus, 

 where it was before, at the anterior end of the blastopore under 

 the dorsal lip (fig. 12). Attention is called to the fact that during 

 all this time the blastopore occupies the ventral portion of the 

 posterior end of the embryo, and that the anus opens posteriorly, 

 not ventrally. 



The backward extension of the dorsal lip of the blastopore 

 sometimes results in the production of abnormalities, owing to the 

 fact that it remains free from the yolk. 



One of the most common of these is seen in fig. 10. Here in- 

 stead of growing downward over the yolk plug as is usually the 

 case, the tail of the embryo has turned upward and grown directly 

 away from it. This is much more common in Rana than in 

 Amblystoma and is probably due to the fact that the cells on 



