320 
E. W. MACBEIDE. 
over tlie lower pole and eventually meets the dorsal lip and 
so the blastopore is closed. In none of the cases discussed 
can there be a question of concrescence of the lateral lips of 
the blastopoi’e extending along the neural plate, because 
unless such concrescence is to be merely a metaphysical fig- 
ment, it must mean that the dorsal lip advances by a pair 
of growing points situated to the right and left of tlie 
median line, whilst in the mid-dorsal line itself there is a 
cessation of growth. Now no such cessation is observable. 
A groove found in the floor of the neural canal in Amphibia 
has been interpreted as a remnant of this concrescence, but 
Brachet has pointed out that this groove is a late secondaiy 
phenomenon that does not make its appearance till the blasto- 
pore is closed.^ From Am ia we pass naturally to the develop- 
ment of a Teleost such as Mormyrus, so well described by 
Assheton (3). Here there is, as in Amia (and, of course, in 
all Teleostei), a well-marked germinal disc. On the surface 
of this the neural plate of the embryo becomes marked out 
by its greater thickness. But this neural plate, instead of 
extending as in the cases we have just discussed over 180° 
or more of the circumference of the egg, extends over quite 
a small arc, so that the embryo is reallj’^ differentiated from 
but a small part of the egg. At the margin of the germ- 
disc as before we find the dorsal lip of the blastopore, but 
the ventral lip sweeps completely round the egg and its two 
sides close by lateral concrescence on the hinder aspect of 
Avhat afterwards becomes the yolk-sac. After this closure 
has taken place, and not till then, the growth in length of 
the hinder part of the enibryo begins. The alimentary canal 
becomes grooved off from the surface of the yolk-sac as a rod 
of cells, and the yolk-sac is slowly absorbed. 
In the development of Elasmobranchs a very small germ- 
disc is formed on the surface of a large yolky egg. Inside 
‘ It is true that Brachet in a later papier (7) admits that he is an 
adherent of the concrescence theory, but solely on the ground of the 
appearances presented by pathological emljryos, which is, I think, a 
most illogical position. 
