80 MALL. [VoL. XIX. 
which the effect upon the embryonic mass was not so marked 
and show to what extent the yolk sac can endure hardship. 
The degree of change is expressed pretty well by the size and 
age of the ova; in the younger ones (Nos. 21, 130 and 123) 
the yolk sacs are simply detached, while in the older ones 
(Nos. 264 and 14) they are fibrous and well attached to the 
chorion. 
The amount of magma within the ccelom tells the same 
story. There is some in No. 21, considerable in No. 130, 
much in No. 123, hard and hyaline in No. 264, and completely 
filled in No. 147. Thus we have in these specimens the 
gradual changes in the umbilical vesicle after the amnion and 
embryo have been destroyed. No doubt some of the earlier 
stages (Nos. 21 to 180) would have reached a stage similar 
to No. 147 had the ova not been aborted. Instead, the yolk 
sacs would probably have been destroyed entirely to make 
chorions without embryos and uterine moles. But few of 
them could go on degenerating for eighty-nine days, ending 
with an atrophic umbilical vesicle. 
The first specimen of this group:is composed of two vesicles 
with blood islands in the outer layer of mesoderm. Of course 
it is possible that one of these represents the amnion, but I 
am of the opinion that it is a dilated allantois on account of 
its close resemblance in structure and layers with the main 
vesicle. Another free umbilical vesicle is found in No. 
130. However, it is uncertain whether or not this was torn 
away from the main embryonic mass before or after the 
ovum was aborted. No. 123 is from a clear case of complete 
separation of the umbilical vesicle from the chorion. The 
ovum appeared normal, but more careful observation showed 
that it was encircled with pus. Within the ccelom the free 
~ umbilical vesicle was found to have a large opening on one 
side, showing that its destruction had also begun. 
No. 180 is another case of entire destruction of the 
embryo, leaving only the umbilical vesicle. The chorion con- 
tains villi and blood-vessels. Between them there is a slimy 
mass, in which there are many leucocytes and islands of 
