92 G. CARL HUBER 



is evident in the albino rat. Indeed, Sobotta presents the 

 far-reaching conclusion that the explanation of the phenomenon 

 of germ layer inversion or entypy of the germ layers is to be 

 found in the dearth of food suppty of the ovum in the stages 

 preceding the formation of more definite relations between the 

 ova or germ vesicles with the decidua. It is thought by this 

 observer that the inversion of the germ disc has for its purpose 

 the increase of the absorptive surface of the visceral or yolk sac 

 entodermal epithelium, which as a differentiated layer comes 

 to surround nearly the whole of the egg-cylinder on comple- 

 tion of the inversion, and is thus increased in extent and brought 

 in relatively close relation with the maternal blood lacunae 

 surrounding the egg vesicle. 



LATE STAGES IN EGG-CYLINDER DIFFERENTIATION AND 

 THE ANLAGE OF THE MESODERM 



In the rat series there are found 24 egg cylinders showing the 

 stages of development considered in this section; certain of them 

 are cut longitudinally and others cross-wise. 



For the special consideration of egg-cylinder formation just 

 prior to the anlage of the mesoderm, I present two egg-cylinders 

 obtained during the latter half of the ninth day after insemination; 

 one of these was cut longitudinally, the other in favorable cross- 

 section. The egg-cylinder shown in figure 29, rat Xo. 40, 8 

 days, 17 hours after insemination, seems unusually well fixed, 

 as evidenced by its symmetrical outline, and is cut in a very 

 favorable plane. The sections are from a series cut at right an- 

 gles to the long axis of the uterine horn. The decidual crypts 

 lodging the egg-cylinders of this stage are by this time nearly 

 completely separated from the lumen of the uterus, and are 

 surrounded by a well-developed decidua. Extravasated mater- 

 nal blood nearly surrounds such egg-cylinders. 



Fig. 29 Longitudinal, sagittal section of egg-cylinder of the albino rat show- 

 ing the final mesoderm-free stage. X 200. Rat No. 40, 8 days, 17 hours, after 

 insemination; ect.pl., ectoplacental cone or Trager; p.ect., parietal or transitory 

 ectoderm; pr.emb.ect., primary embryonic ectoderm; ex.ect., extraembryonic 

 ectoderm; pr.c, proamniotic cavity; v.ent., visceral entoderm, absorptive for 

 maternal hemoglobin, cells showing the three zones described by Sobotta; 

 pr.emb.ent., primary embryonic entoderm. 



