438 BULLETIN 82, UNITED STATES NATIONAL MUSEUM. 



Only with a similar reservation can the ventral side of the 10-hour embryo 

 be inferred from that of the 16-hour stage, for in intermediate stages the 

 gut often appears straight, so that the surface toward which it inclines at first 

 is not necessarily that to which it will eventually be curved. 



When preparing to divide the ectoderm cells contract and, withdrawing to 

 the exterior of the cell layer, there form large globular cells widely excluded from 

 the primitive body cavity by the meeting of the adjacent prismatic cells beneath 

 them. On sections these neighboring prismatic cells show more or less extensive 

 incisions corresponding in outline to the outline of the globular cell. After division, 

 which occurs in the usual way, the daughter cells elongate and push their way 

 down between the surrounding prismatic cells until their inner ends reach the 

 primitive body cavity, becoming in the process indistinguishable from these. In 

 sagittal sections of the ectoderm which show the prismatic cells as five or six 

 sided polygons their nuclei are sometimes seen undergoing division, so that it 

 may be inferred that it is not always necessary for the ectoderm cells to assume 

 the globular form preparatory to division. 



The entoderm cells differ from those of the ectoderm in their shorter and more 

 rounded form. But among the relatively short and rounded cells there are 

 numerous more slender prismatic cells of the same length as the others which 

 possess two nuclei, resulting from the division of a mother nucleus, one of which 

 lies toward the gut lumen, the other toward the body cavity. The appearance 

 of these two nuclei is followed by the division of the cell body in such a way that 

 one of the daughter cells borders the gut lumen, the other the body cavity. 



These daughter cells bordering the body cavity are first found at the blind 

 end of the primitive gut and represent the mother cells of the mesenchyme. 



Between the typical entoderm cells there are others of approximately equal 

 height, which toward the body cavity are swollen into a very thick, rounded base, 

 and toward the gut lumen are drawn out to a fine point. 



The several-layered condition of the entoderm, which leads to the formation 

 of the mesenchyme, appears to result not only from the division of the entoderm 

 cells parallel to the surface as just described, but -also from the formation and 

 subsequent liberation of these top-shaped elements. 



All stages between the cells just emerging from the entoderm and the typical 

 mesenchyme cells can be found. The process reaching in between the entoderm 

 cells shortens and finally is entirely withdrawn, while at the same time the cell 

 severs all connection with the entoderm. 



The typical mesenchyme cells, which in the primitive body cavity lie abun- 

 dantly on the entoderm, are strongly rounded and invariably show a nucleus. 

 They undergo division individually and independently. 



In the 19-hour embryo, as a result of continued cell division, the ectoderm cells 

 have become shorter and the whole ectoderm layer therefore thinner. As the 

 embryo has increased not at all or only insignificantly in size, additional ectoderm 

 cells must have forced themselves through the blastopore opening, since the primi- 

 tive gut sack has become more extensive, reaching quite across the primitive body 

 cavity to the animal pole; indeed, in a cross section through the blastopore a 



