NOTES ON ELASMOBRANCH DEVELOPMENT. 561 
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 ventralwards 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 
1 A good figure of this is given by His in the ‘ Zeitschrift f. Anatomie u. 
Entwick. Gesch.,’ 1877, pl. vii, fig. 6. 
