DEVELOPMENT OF THE ALBINO RAT 307 
thin-walled roof of this vesicle, the parietal or transitory ecto- 
derm, deserves no special consideration, except to state that its 
variation in thickness, as seen in the section figured, is due to 
the plane of section—cross or oblique—of different portions of 
the wall, owing to slight folding. This vesicle I believe to be 
in stage of development and structure very similar to that 
shown by Sobotta (’03) in his figure 6, mouse vesicle of the first 
half of the sixth day, and perhaps also figure 31, of the account 
of Melissinos, mouse vesicle, end of fourth day, also figure 7 
of Jenkinson’s article who, however, describes and figures a 
distinet covering or trophoblast layer. 
The cell rearrangement and proliferation resulting in the 
thickening of the floor or the germinal disc as noted in E, of 
figure 23, marks the beginning of a much more distinet thick- 
ening of this portion of the vesicle, partly due to cell proliferation, 
in part also due to the rearrangement and enlargement of the 
constituent cells, during which thickening process this portion 
of the vesicle grows outward as well as into the cavity of the 
vesicle, initiating the phenomenon known as the ‘inversion of 
the germ layers’ or as ‘entypy’ of the germ layers, to be discussed 
as to its anlage in the following section. 
LATE STAGES OF BLASTODERMIC VESICLE, BEGINNING OF 
ENTYPY OF GERM LAYERS 
The material at hand is listed in table 7. 
TABLE 7 
RECORD NUMBER AGE NUMBER OF VESICLES 
Ao Saalte PER RA 2 [24s 2 
46 6 days, 14 hours | 10 
54 6 days, 16 hours | 9 
67 6 days, 16 hours ih 
24 | 6 days, 17 hours 3 
90 6 days,.17 hours | 6 
72 7 days | 9 
80 7 days | 9 
92 7 days 6 Total 59 
The fixation of the blastocysts of the albino rat obtained 
during the seventh day after insemination was much more 
