POLYEMBRYONIC DEVELOPMENT IN TATUSIA 583 
The center of the embryonic spot lies about five millimeters be- 
low the center of the circular figure, and, since the entodermal 
cells are practically all in focus, it can be seen that, as a continu- 
ous layer, the entoderm covers only about the lower three-fourths 
of the embryonic spot. Over the other fourth only a very few 
entodermal cells are found, and these lie for the most part slightly 
to the left of the center. The other cells in this area which are 
slightly out of focus represent the exposed ectoderm, which is here 
only about one cell deep. | 
What does the excentric position of the entoderm mean? The 
observation of this phenomenon in some four or five blastocysts 
doubtless furnishes too meager evidence upon which to base any 
fundamental conclusion. Nevertheless one can not resist the 
temptation to suggest that we may have here a key to the much 
mooted question of gastrulation in eutherian mammals. For if 
it could be shown that the origin of the entoderm is confined to a 
definite area of the embryonic mass, the center of such an area 
might be regarded as corresponding to the region of a blastopore, 
regardless of whether or not this spot later became perforated by 
an actual opening or evanescent blastopore. Hubrecht (’02 ’05 
’08) has argued that the didermic stage of the mammalian blasto- 
cyst is to be regarded as a ‘gastrula’. He further states that we 
must separate the phenomena of notogenesis from the phenom- 
enon of gastrulation. He expresses himself very clearly and 
concisely on this point in the last of the three contributions 
mentioned above in which he makes the following statements: 
As soon as we separate the phenomena of notogenesis, such as we have 
found in all vertebrates—Amphioxus included—from the phenomenon 
of gastrulation, recognizing that the former follow upon the latter and 
bring about the formation of the notochord and the mesoblastic somites, 
the difficulties are considerably simplified. 
Gastrulation is thus terminated in the mammalia when the didermic 
stage of the embryonic shield has come into existance. We have seen 
that this takes place not in consequence of any process of invagination 
but by means of a most unmistakable delamination of the entoderm, out 
of the embryonic knob. 
This delamination gastrula of the mammalia generally enters upon the 
latter phases of ontogeny which will be described hereafter without the 
appearance of a distinct blastopore.* 
4\Loe. cit.;,.p. 13. 
