96 G. CARL HUBER 



The egg-cylinder presented in figure 29 constitutes the final 

 mesoderm-free stage, the final stage in which no distinct bilateral- 

 ity may be determined. I assume that the egg-cylinder pre- 

 sented in the figure is cut in the sagittal plane. This assumption 

 is based on the fact that the primary embryonic ectoderm ex- 

 tends slightly farther toward the mesometrial pole on the one 

 side than on the other. In good frontal sections one side of the 

 egg-cylinder in this stage of development should present a mir- 

 ror picture of the other side. The side on which the primary 

 embryonic ectoderm extends farther toward the mesometrial 

 pole, the left in the figure, is regarded as containing the caudal 

 end of the future embryo. In the primary embryonic ectoderm 

 of this region, it is believed, will develop the primitive streak and 

 groove, and thus the anlage of the mesoderm. Not in all the 

 egg-cylinders of this stage of development found in my series can 

 the caudal end of the future embryonic area be postulated prior 

 to the anlage of the mesoderm, and in cross-sections no such 

 differentiation can be made. The proamniotic cavity of the 

 egg-cylinder shown in figure 29 presents a regular and nearly 

 smooth contour, not divisible into regions such as described 

 for a similar stage for the mouse by Selenka, Melissinos, and 

 Sobotta. A very slight constriction is to be observed only in 

 the region where the primary embryonic and extraembryonic 

 ectoderm are joined in a continuous layer. I am thus wholly 

 in accord with Widakowich, who in describing a similar stage in 

 one of his preparations, states: "Das war die einzige konstante, 

 bald starker, bald schwacher ausgepragte Einschniirung der 

 Proamnionhohle," as previously quoted. 



A series of figures of critical regions taken from a series of 

 cross-sections of an egg-cylinder of a stage nearly identical with 

 that shown in figure 29, though of a slightly smaller egg-cylinder, 

 is given in figure 30, rat No. 42, 8 days, 16 hours, after insemina- 

 tion. The sections chosen for the several drawings, A to D, are 

 from the following regions, as may be ascertained by compari- 

 son with figure 29; A, through about the middle of the ectoplacen- 

 tal cone; B, through the proamniotic cavity just below its meso- 

 metrial end; C, through the proamniotic cavity just above the 



