DEVELOPMENT OF THE ALBINO RAT 63 



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 thai 

 shown by Sobotta ('03) in his figure (>, mouse vesicle of the firsl 

 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 

 distinct 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 distinct 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 he 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. 



The fixation of the blastocysts of the albino rat obtained 

 during the seventh day after insemination was much more 



