350 W. L. SCLATER. 



be part of the uterine epithelium and a purely uterine struc- 

 ture, whereas to me it seems evident that the wall of the 

 embryonic vesicle has nothing whatever to do with the uterine 

 epithelium, but is derived solely from the outer layer of the 

 pseudogastrula exactly in the same way as the surrounding 

 layer of the mammal's blastoderm, which afterwards forms the 

 chorion, is derived from the outer cells of the Mammalian 

 pseudogastrula. 



From the pseudogastrula stage onward the embryo is always 

 found lying in a hollow space, the embryonic vesicle, and 

 attached on one side to the wall of the embryonic vesicle, which 

 is formed from the outer layer of the pseudogastrula stage. 



The embryo during this stage is at first sessile, after- 

 wards a stalk is formed ; and it is during the formation of the 

 stalk that the two structures termed by Kennel ammion and 

 placenta are found. 



Figs. 8 and 11 represent the youngest embryos in vesicles 

 which I have met with. 



Fig 11 is drawn from an embryo lying in its vesicle still in 

 the uterus, the uterus has been slightly split and the object 

 drawn after being rendered transparent in benzole. The most 

 noticeable point about fig. 11 is the enormous increase in size 

 of the vesicle which is represented in the previous stage only 

 by the slight split between the inner and outer layers of the 

 pseudogastrula ; the embryo, which is seen to form a small dark 

 patch on one side of the vesicle, is oblong in shape and sessile. 

 The size of the embryo is -16 mm. long by -06 mm. across; 

 this is approximately of the same size as the embryo itself in 

 the previous stage; but the vesicle in figure 11 measures "24 

 mm. across as against *11 mm. in the pseudogastrula stage 

 (fig. 7 b). 



Fig. 8 shows a section through an embryo measuring •084 mm. 

 in diameter with part of its vesicle of the same age as fig. 11. 

 The wall of the vesicle consists of a band of clear protoplasm 

 with definite nuclei at intervals. The embryo itself is composed 

 of two parts, the basal part, in which the nuclei resemble those 

 of the vesicle wall, and the embryo proper, consisting of large 



