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 

 on-ly 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 



4 Loc. cit., p. 13. 



