136 G. CARL HUBER 



beginning of insemination. The uterus of this rat contains 

 eight egg-cylinders, all of which, except the one here figured, 

 show normal structure, though presenting quite different stages 

 of development. One of these, cut serially in cross-section, is 

 figured in C, figure 32, Part I, as showing anlage of mesoderm 

 with primitive streak and groove. Two of the other egg-cylinders 

 show the anlage of the mesoderm, two others show late pre- 

 mesoderm stages of the egg-cylinder, the remaining egg-cylinders 

 are less fully developed, one showing a development which may 

 be compared to B of figure 26, Part I, thus a much younger 

 stage. By the end of the eighth day and with the early hours 

 of the ninth day after the beginning of insemination in the 

 albino rat, the two parts of the proamniotic cavity, which de- 

 velop discretely, have joined to form a single space (C, fig. 27, 

 Part I). The egg-cylinder shown in figure 9, presents normal 

 development in all parts, except that there is as yet no union of 

 the two parts of the proamniotic cavity. This egg-cylinder is 

 most favorably cut, in longitudinal direction; the plane of sec- 

 tion being almost parallel to the mid-sagittal plane. This egg- 

 cylinder, therefore, is easily followed through the several sections 

 of the series into which it was cut. The irregularity of outline 

 of the ectodermal vesicle, lower right of figure, it is believed, is 

 not due to fixation shrinkage. Judging from size and structural 

 differentiation of this egg-cylinder, union of the antimesometrial 

 and mesometrial portions of the proamniotic cavity should have 

 been completed before this stage of development was reached, 

 with the primary embryonic ectoderm and the extraembryonic 

 ectoderm forming a continuous layer, as shown in figure 29, Part I. 

 The folding of the wall of the antimesometrial portion of the 

 egg-cylinder, lower right of figure, evident in nearly all of the 

 sections of the series, is regarded as indicating an abnormal 

 growth of the primary embryonic ectodermal cells composing the 

 wall of the ectodermal vesicle, as a result of retarded extension 

 of the antimesometrial portion of the proamniotic cavity, perhaps 

 an adjustment to meet the altered mechanical stress resulting 

 from abnormal development. The condition here seen, it would 

 seem, is foreshadowed in the egg-cylinder shown in figure 8. 



