622 J. T. PATTERSON 



(fig. 33), and there is gradually evolved a distinct epithelial 

 layer (figs. 33, 34, 64). Following the suggestion of Fernandez 

 ('09) in his paper on Mulita, we may designate it the 'Trager 

 Epithelium/ on contrast to the Trager zone, or thickened an- 

 nular portion of the trophoblast which forms the base of the at- 

 tached chorionic vesicle. The Trager epithelium, together with 

 the layer of extraembryonic mesoderm which directly overlies it, 

 forms the lower wall of the chorionic vesicle. 



In the meantime the deeper-seated nuclei continue to migrate 

 farther into the uterine tissue, which, as already stated, soon 

 becomes destroyed. During these changes the upper portion of 

 the syncytium begins to degenerate, as is evidenced by the appear- 

 ance of numerous vacuoles (figs. 34, 35). The vacuoles flow 

 together and eventually produce a shallow cavity which lies 

 just beneath the now well-organized Trager epithelium. The 

 final condition in the development of this 'sub-Trager cavity' is 

 well illustrated in the series of photographs in plate 7. All of 

 these show the condition just referred to, but figure 64 is par- 

 ticularly clear. Here the sub-Trager cavity is sharply defined 

 above by the Trager epithelium, but on the lower side it is con- 

 nected with numerous spaces which lie between the remains of 

 the partly dissociated mucosa. 



Occasionally during the process of fixation the Trager epithe- 

 lium becomes pushed up into the extraembryonic cavity, carry- 

 ing before it the layer of mesoderm. Under such conditions it 

 appears as though the Trager epithelium was normally arched 

 (figs. 66-69). I am inclined to believe that a similar artificial 

 condition in the chorionic vesicle of Mulita has lead Fernandez 

 ('09) to conclude that the Trager epithelium is normally pushed 

 up into the extraembryonic cavity, and that consequently the 

 cavity lying just below it is to be regarded as a true Trager cav- 

 ity. In view of the fact that Fernandez had only a few scatter- 

 ed stages at command, it was easy for him to be mislead into 

 drawing this conclusion. But the evidence which I have just 

 presented from a study of the development of this cavity makes 

 it doubtful whether it should be regarded as a Trager cavity, in 

 the sense in which that term is used in rodents. For that 



