336 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 supply 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 No. 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. > 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; ew.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. 
