POLYEMBRYONIC DEVELOPMENT IN TATUSIA 589 



not be doubted, and this effect seems to be limited entirely to the 

 region in direct contact. 



The sections are cut somewhat obliquely, and hence the section 

 passing through the middle of the embryonic ectoderm does not 

 cut the region of the primitive placenta, or point of attachment 

 (fig. 16). Consequently the exact relation of the entoderm to the 

 other parts of the blastocyst can only be made out by referring to 

 several of the sections of the series. In plate 2 is shown a series 

 of four photographs which will demonstrate this relation. 



In figure 43, which represents the section from which the draw- 

 ing was made (fig. 16), the entoderm is a well organized layer pass- 

 ing around the sphere of ectoderm. On approaching the low- 

 er side of the ectoderm the entoderm at each side passes laterally 

 and upwards to join on to the inner surface of the trophoblastic 

 wall (fig. 16, x). The layer of entoderm lying directly beneath 

 the ectoderm belongs to that portion which connects the entoder- 

 mal sac with the left wall of the trophoblast. A few sections to the 

 right in the series the lower layer of entoderm disappears, except 

 a small group of cells (fig. 44) . In this section the entoderm, on 

 leaving the ectoderm, passes outwards and upwards as before,to 

 join the inner surface of the trophoblast. Aside from the group of 

 entodermal cells lying directly beneath it, the embryonic ectoderm 

 is open below to a cavity which is bounded beneath by the placental 

 portion of the trophoblast, and which represents what is later to 

 become an extraembryonic cavity. In sections still farther to the 

 right, at a point where the section cuts just the tip of the ecto- 

 derm (fig. 45), the extraembryonic cavity is almost free from cells 

 of any kind. This section passes through the center of the Trager 

 (fig. 31). Beyond the limits of^the ectoderm (fig. 46) we see noth- 

 ing but a line of entodermal cells, which represents the right lat- 

 eral portion of the entodermal layer as it passes outwards to unite 

 with the trophoblast on this side of the blastocyst. 



To sum up: The conditions revealed in the series of photo- 

 graphs shows that the ectoderm, upon assuming a spherical form 

 pushes up into the cavity of the blastocyst, carrying before it the 

 well established layer of entoderm, and creating behind a cavity 

 which gives rise to the extraembryonic cavity, and which will 



