142 
(p. 281) that “une partie importante de la plaque médullaire 
siège en arriêre du blastopore”, as follows from fig. 2. 
This seal-ring or crescent is evidently to be derived by 
one-sided development from a ring of uniform breadth as 
represented by the stomodaeal rudiment in Protaxonia. 
uring the contraction of the blastopore this ring-shaped 
rudiment of the medullary tube undergoes a change in 
shape as indicated in figs. 36 f and 39. We see here the 
backward growing out of the stomodaeum of Annel:ds into 
the epichordal neural tube of Chordates projected as it were 
on a plane, this being the dorsal side of the embryo. Thus in 
ontogeny it is not the neural tube itself that grows out but 
its rudiment, before the tube has been formed. This may at 
med É an j 
Er 
(p.neurent:) 
a. b. Ë d. 
Fig. 39. Formation of the epichordal part of the medullary tube in 
Chordates. In d the medullary folds are closing. 
first appear a little strange and it might easily be 
supposed that it is the neurotroch of the Annelid that 
has simply sunk under the surface and that in thís way 
the ciliated neural tube has been formed. By this assump- 
tion, however, the fate of the blastopore, its backward 
movement and its conversion into the neurenteric canal, 
would not be accounted for. Moreover, we shall find our 
conception confirmed when considering the relation of the 
anus to the blastopore and the formation of the tail. _ 
As we have seen there is a certain genetic relation 
between the neurotroch and the stomodaeum in Annelids 
(cf. fig. 37) and there is reason to consider the former 48 
an obliterated part of the latter As both consist of ciliated 
and vacuolated cells, there is also a certain histological 
agreement. If now the blastopore moves backwards In 
Chordates it is quite possible, and even probable, that 
the cells of the neurotroch are reincorporated into the 
material for the stomodaeum, now the medullary tube. Then 
there appears to be still a certain relation between the 
