DEVELOPMENT OF THE ALBINO RAT 305 
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 
