DEVELOPMENT OF GERMINAL LAYERS IN MAMMALS. 429 
to a modified archenteron. The non-appearance of the seg- 
mentation cavity is probably due to the rapid formation of a 
comparatively large mass of hypoblast which has invaded the 
region in which the cavity would have appeared under other 
circumstances. Looked at from this point of view, the ova of 
the animals in question present the peculiar features of all 
comparatively large-yolked ova; that is, they consist of a small 
mass or layer of epiblast superposed upon one pole of a large 
hypoblastic mass, and consequently in these ova, as in all ova 
with a comparatively large yolk-mass, the formation of the 
archenteron commences before the epiblast has extended over 
the whole surface of the hypoblast. 
The very early stages of the development of the shrew’s ova 
have not been observed; but the blastocyst of that animal is 
so similar to the blastocyst of the mole that there can be but 
little doubt that the changes which occur during the very early 
stages of development in the ova of both these animals are very 
similar. 
The phenomena which have been noticed in the opossum 
seem at first sight to point to the conclusion that the segmenta- 
tion cavity becomes the archenteron ; but it is to be noted 
that between the very early stages in which the segmentation 
cavity is present and the stages of a distinct blastocyst there 
is a hiatus in the specimens observed, and it is not at all im- 
probable that the stages intervening are those in which the seg- 
mentation cavity is obliterated and the archenteron is formed. 
If the explanation I have suggested is correct, it is interesting 
to note that in the hedgehog we have comparatively a small 
amount of hypoblast, which is rapidly surrounded by epiblast, 
and the presence of both segmentation cavity and archenteron ; 
that inthe rabbit, mole, shrew (?), and the bat there is a relatively 
large mass of hypoblast, which is only gradually surrounded 
by the epiblast, and there is no segmentation cavity ; that in 
the opossum there is a relatively large amount of hypoblast, 
but a segmentation cavity appears; and that in the rat and 
mouse, on account of special circumstances, the epiblast does 
not completely surround the hypoblast. In other words, we 
