STUDIES IN MAMMALIAN EMBRYOLOGY. 485 



Beginning with the processes in the maternal tissue^ 

 we notice that the distensions above referred to change the 

 aspect of the uterine horns, which were originally cylindrical 

 ducts, most considerably. 



Not only in this sense, that spherical swellings indicate the 

 advance of pregnancy, as in other mammals, but the distensions 

 take a definite pear shape very soon after the embryo has wan- 

 dered from the oviduct into the body of the uterus. These 

 swellings are first connected by portions of great tenuity, which 

 only widen in the later stages of pregnancy. 



This distension is undoubtedly independent of any direct 

 action of the blastocyst. It is a maternal phenomenon prepa- 

 ratory to the adhesion of the blastocyst. Simultaneously with 

 it the epithelial lining of the lumen of the distended portion 

 undergoes important changes. 



These changes consist in a rapid increase of the epithelium 

 cells, which become more than one layer thick, and between 

 which vascular channels are enclosed, the final result being — 



1°. A concave bell-shaped surface opposite the mesometrium, 

 on which numerous newly formed epithelial crypts open out, 

 gland openings being here and there interspersed between the 

 much more numerous mouths of the crypts. 



2°. Lateral cushion-shaped surfaces, where no similar crypts 

 are present, and against which the blastocyst first adheres by 

 means of a zonary strip. 



The embryonic processes are the following : — The blood- 

 vessels of the area vasculosa on the yolk-sac spread out against 

 this zonary strip, those of the allantois (soon ramified in digitate 

 villi) against the concave surface referred to sub. 1° ; not, 

 however, before most important changes have taken place in 

 these two regions — changes that consist in the development 

 of a syncytial tissue of embryonic origin out of the outermost 

 layer of the blastocyst. 



In this embryonic syncytium two regions — the counterparts 

 of the maternal proliferations above referred to — may be 

 distinguished : 



1. A zonary syncytium in the region of the area vasculosa. 



