EARLY ONTOGENETIC PHENOMENA IN MAMMALS. 53 
cavity which in the mammal is styled the umbilical vesicle, 
and which is the so-called archenteron in the Amphibian. 
The concrescence between this and the segmentation cavity 
is the same as is noticed at the yet earlier stage of Tarsius 
(Fig. 19), in which the entoderm forms the roof of the 
trophoblastic cavity. But we will return to these possible 
comparisons later on. 
It remains to be seen whether in other Amphibia than 
Rana and Triton the presence of a fourth focus from whence 
tissues are originated that take their place between ecto- and 
ento- derm can be confirmed. In other words, whether any- 
thing corresponding to the annular zone of mesenchyme- 
producing entoderm (stretching backwards right and left 
from the protochordai plate and reuniting in the median line 
posteriorly under the ventral mesoblast) as it was figured in 
Fig. 60 occurs in Amphibia. 
Although Brachet has not expressly stated that such an 
annular zone of entoderm was noticed by him, we may 
conclude from his descriptions that it does occur in his 
preparations. On p. 88 (’03) he writes about: “ L’intense 
activité que l’on pourrait appeler mésoblastogéne des cellules 
de la voute”’ (by which latter he means the roof of the 
archenteron); and on p. 89: ‘‘ Les bandes mésoblastiques 
sont plus épaisses dans la région blastoporale que dans la 
région gastrale proprement dite ... Le mésoblaste péri- 
stomal est beaucoup plus abondant que le mésoblaste gastral 
(p= 90)” 
From these citations I think we may conclude that the 
presence of an annular zone of mesenchyme-producing 
entoderm in Amphibia will in due time be yet more fully 
established. 
Authors who have actually figured it in the posterior 
median line of the embryonic Anlage are Robinson and 
Assheton (91, Figs. 14—17), where in the median region 
of the blastopore and behind it we notice an entodermal 
proliferation producing what the authors call ‘ hypoblastic 
or inner layer of mesoblast of primitive streak,” as against 
