324 
E. W. MACBRIDE. 
these facts, it has been inferred that the columnar epithelium 
of the invaginated sac is ectoderm, and even when this sac 
has opened into the “segmentation” cavity it is inferred that 
a sharp line can be drawn between the two kinds of cells. 
Now with reference to this development there are several 
points to be noted. First the description labours under the 
disadvantage of being founded on a limited number of 
specimens. Braner found great difficulty in getting his 
material, and his specimens of young stages were few. In 
particular a gap occurs about the time of the “breaking 
through ” of the invaginate archentei’on, and he has to admit 
that it is difficult to fix the precise spot where one sort of 
cell leaves off and the other begins. There is no proof that 
the place where the “undergrowth” begins corresponds to 
this point. Next the invaginate cells are different from the 
ectoderm in several points, and in any case a descilption of a 
process founded on scanty specimens in which considerable 
gaps have to be bridged cannot hold against one like that of 
Brachet of Siredon founded on an enormous mass of material. 
The whole thing can be interpreted more simply as follows : 
lirauer admits that the dorsal lip grows backwards; it is 
therefore to be regarded as a growing point from which new 
cells are added to the ectoderm above and to the endoderm 
below. But we saw that in other Amphibia, before any 
invagination had taken place or a dorsal lip had grown out, 
the archenteric cavity appeared as a cleavage amongst the 
large cells at the lower pole of the egg. Now if we assume 
that this cleavage i s represented in Hypogeophis by 
the so-called segmentation cavity {/), which, however, 
happens to form, not at the surface of the egg, but further in, 
and that the “invagination” is the formation of new endo- 
derm from the growing point in the dorsal lip, we shall be able 
to completely reconcile the development of Hypogeophis 
with that of Amphioxus and of Siredon. Indeed, in the 
Sturgeon’s egg, as figured by Deane and reproduced in text- 
figs. 5, 7 and 8, the archenteric cavity is almost divided into 
two parts, such as I have described. The complete separation 
