134 G. CARL HITBER 



ing its absorptive function. The extraembryonic ectoderm, 

 enclosing the mesometrial portion of the proamniotic cavity, 

 presents normal structure and relations of cells. The only ab- 

 normality observed is in the region of the ectodermal node, the 

 anlage of the ectodermal vesicle with the enclosed antimesometrial 

 portion of the proamniotic cavity. With this stage of develop- 

 ment of the egg-cjdinder (see figs. 26 and 27, Part I) the ectoder- 

 mal node presents a well formed cavity, surrounded by the cells 

 of the primary embryonic ectoderm, radially arranged. In the 

 egg-cylinder under discussion (fig. 8) there is distinctly a retar- 

 dation in the development of the ectodermal vesicle with full 

 differentiation of the primary embryonic ectoderm. An imper- 

 fectly developed antimesometrial portion of the proamniotic 

 cavity is evident. This small cavity, indistinctly bounded, ex- 

 tends obliquely through several sections of the ectodermal node, 

 and contains amorphous granular detritus, which in the prepa- 

 rations is stained by Congo red. The cells destined to form the 

 primary embryonic ectoderm show no definite arrangement, 

 especially as concerns the more centrally placed cells of the 

 node. Since the primary embryonic ectoderm is the anlage 

 for the ectoderm of the embryo, an arrest in its differentiation 

 would of necessity profoundly affect further development of 

 the embryo. Antimesometrial to the ectodermal node (just 

 above it in the figure) there is found a small vesicle the walls of 

 which are not distinctly delimited and composed of extraem- 

 bryonic ectodermal cells, surrounding a small, completely bounded 

 cavity. I am not prepared to say whether this small vesicle is 

 to be regarded as developing from cells of the extraembryonic 

 ectoderm, or from a displaced, accessory ectodermal node, in 

 which a discrete portion of the proamniotic cavity has developed. 

 If the latter, the possibility of a double anlage for the embryonic 

 ectoderm is to be considered. My interpretation of this egg- 

 cylinder as showing a retardation of the development of the 

 ectodermal node and differentiation of the primary embryonic 

 ectoderm, is confirmed from a study of a slightly older stage show- 

 ing essentially the same condition. This ovum is presented in 

 figure 9, and is taken from rat No. 41, 8 days, 16 hours, after the 



