236 ADAM SEDGWICK. 



extent with the notochord, though the notochord beneath the 

 front part of it is not at first developed.) 



It must be clearly understood that the growth of the whole 

 edge of the blastoderm has so far been a uniform one. The 

 indentation in the embryonic rim advances equally (after its 

 first establishment) with the more prominent parts of the 

 embryonic rim called the caudal swellings. There is no 

 reason to suppose that this advance of the indented part of 

 the embryonic rim is due to the fusion of the divergent caudal 

 swellings. On the contrary, there is every reason to suppose 

 that the indented part of the embryonic rim advances by 

 growth of its own substance, just as do the other parts of the 

 edge of the blastoderm. 



After a certain time the caudal swellings and the part 

 between them begin to grow more rapidly than the adjacent 

 portions of the edge of the blastoderm, and come to project 

 beyond the latter like a kind of tongue overhanging the yolk 

 (fig. 2). This appears to happen at about the time when the 

 medullary groove is closing in its anterior part to form the 

 medullary canal. 



At the same time the edge of the blastoderm remote from 

 the embryo has continued its rapid growth. It is only the 

 edge of the blastoderm next the embryo in which the growth is 

 retarded. The result of this is that the posterior projecting 

 part of the embryo lies in a kind of bay of the edge of the 

 blastoderm. Fig. 2 is drawn from an embryo at a stage when 

 this bay was but little marked. 



I now wish the reader to concentrate his attention upon the 

 projecting tongue which will form the under part of the 

 embryo. Its sides, which are part of the edge of the blasto- 

 derm, bend ventral wards and towards each other .^ It consists 

 on its dorsal face of the medullary plate ectoderm, which has 

 become folded so as to form the neural canal (in fig. 2 the 

 neural canal is established in the front part of the embryo, but 

 widely open at the hinder end of this projecting tongue). At 



' A good figure of this is given by His in the ' Zeitschrift f. Auatomie u. 

 Entwick. Gesch.,' 1877, pi. vii, fig. 6. 



