DEVELOPMENT OF THE ALBINO RAT 61 



minal disc, having all essentially the same structure, are of 

 irregular polyhedral form and are mutually compressed. To 

 designate them as a distinct germ layer at this stage seems 

 inappropriate. A differentiation into a layer of covering cells 

 and a layer of formative ectoderm (Selenka) is not to be made. 

 Active cell proliferation as evidenced by mitotic figures does not 

 appear to accompany this enlargement of the vesicle. This 

 phenomenon seems rather to be accomplished by a rearrange- 

 ment of the cells constituting its floor, however, primarily by 

 an extension and consequent flattening of the cells forming 

 the roof of the vesicle. A similar stage is shown for the mouse 

 by Sobotta ('03) in his figures 3, 4, and perhaps 5, of mouse vesi- 

 cles from the fifth day after fertilization 'Befruchtung'. So- 

 botta had at his disposal much more perfectly fixed vesicles than 

 my material contains. The structure of these vesicles as given 

 by this observer, both as depicted in figures and text, is very 

 similar to the presentation given by me. He also recognizes 

 in this stage the anlage of the yolk entoderm. Figure 30, ac- 

 companying the account of Melissinos (mouse, 84 hours) presents 

 a similar stage, although he figures fairly distinctly a layer of 

 covering cells, which if I read him correctly, however, is of only 

 transitory existence. None of the figures given by Robinson 

 and Jenkinson is comparable with figures A, B, C, of figure 23 

 of this account. 



In D, of figure 23 (rat No. 100, 6 days) there is reproduced a 

 section of a blastodermic vesicle which on superficial study 

 presents a somewhat later stage of development than those 

 shown in A to C, of this figure. It is, however, only very slightly 

 older than the three vesicles discussed. Vesicle D, cut in good 

 longitudinal direction, is in reality much more folded than ap- 

 pears from the section figured. Its floor or germ disc is com- 

 pressed in a plane parallel to that of the plane of section, so that 

 the germinal disc is cut obliquely and not transversely, and 

 thus appears thicker in the section than it in reality is. A dis- 

 tinct layer of covering cells, continuous with the cells of the 

 parietal ectoderm, is evident. Such a layer of covering cells is 

 figured by Selenka, Jenkinson, and Duval. The yolk entoderm 

 has differentiated and extends by perhaps three cells, in the 



