54 G. CARL HUBER 



figure 20 and A of figure 22. According to this observer, the 

 segmentation cavitj^ arises as a single space, due to an accumula- 

 tion of fluid secreted by the cells of the morula. This secretion 

 is evidenced by globule-like droplets which are shown in his 

 drawings as adhering to certain of the cells bounding the seg- 

 mentation cavity. In my own preparations of the albino rat, 

 I find no evidence which would lead to the supposition that the 

 segmentation cavity does not begin as a single space nor do I 

 find any evidence of secretory globules as described bj r Melis- 

 sinos. Selenka has described quite fully two blastodermic 

 vesicles of the mouse, lying free in the uterine lumen. His 

 account of their structure, supported by two somewhat dia- 

 grammatic figures, is as follows: The wall of the vesicle is formed 

 by a layer of covering cells — 'Deckzellen' — constituting a cover- 

 ing layer — "Deckschicht or Rauber's laj^er." The space en- 

 closed by this layer, for about one-third to one-half of its extent, 

 contains the 'formative cells,' for the remainder it contains 

 fluid. The covering cells and formative cells are said to be sep- 

 arated by a sharp line. The formative cells are in all parts 

 separable into two fundamental germ layers. An inner layer, 

 bordering the cavity and constituting the entoderm, is said to 

 be composed of cells possessing more deeply staining nuclei, 

 irregular outline, with tongue-like processes which extend into 

 the cavity, and a granular protoplasm; further, of cells which 

 are more clear, more peripherally placed, and which constitute 

 the ectoderm. Each of these fundamental germ layers con- 

 sists of a single layer of irregularly formed cells. According to 

 the observations of Selenka, therefore, the floor of the vesicle 

 consists of three layers of cells; an outer covering laj'er — 'Deck- 

 schicht' or 'Rauber's layer' — an inner layer of entodermal cells, 

 and an intermediate layer of ectodermal cells. Jenkinson's 

 account reads as follows: "There are present (1) an outer layer, 

 one cell deep, of trophoblast, which is continuous over (2) an 

 inner mass which becomes differentiated into the embryonic 

 epiblast and the hypoblast, and which is quite distinct from 

 the overlying trophoblast, as my specimens invariably show." 

 In Jenkinson's figures 1 and 2, giving early stages of the blasto- 

 dermic vesicle, there is not shown a differentiation of the inner 



