STUDIES IN MAMMALIAN EMBRYOLOGY. 303 
decrease in thickness of the first-named region (ef. diagrams 31 
—35), it is easily understood that.its maternal blood-supply is 
considerably diminished, and that the raison d’étre of the ompha- 
loidean attachment must at the same time dwindle away. The 
further continuation of the process here described can be gathered 
from the comparison of diagrams 34 and 35 ; the latter leading 
up to the final stage of reduction which the yolk-sac undergoes 
in the later stages of pregnancy, and which is represented in 
fig. 36. Curious histological modifications of the vitelline 
villi have at the same time come about (figs. 45 and 46). The 
circulation of embryonic blood on the yolk-sac is diminished 
but never ceases entirely, and even in a yolk-sac that remained 
attached to the torn membranes of an after-birth, found loose 
in a uterus after expulsion of the embryo, bright red arterial 
blood was still contained in these vitelline vessels. Frommel 
notices a similar phenomenon in the bat.! The histological 
modifications are more especially expressed in the shape of the 
cells. These increase in size after the villi have been pulled 
out of their trophoblastic sheath and have large nuclei. They 
have an evidently epithelial aspect, and are figured for as late 
a stage as that of the diagrams 35 and 36 in figs. 45 
and 46. Another peculiarity about this remarkable retro- 
gressive metamorphosis of the yolk-sac are the very regular 
longitudinal foids in which the yolk-sac is always found in the 
ripe stages, viz. plaited against the animal’s belly with one 
median and two lateral folds (cf. fig. 36). This folding is, as 
far as I can make out, brought about by the arrest of growth 
of the yolk-sac at a certain stage of its retrograde development, 
combined with regular displacements which the embryo under- 
goes during the successive phases of pregnancy. I will just 
touch upon these displacements in order to explain the folding 
process here referred to. Up tothe first appearance of the allan- 
tois the embryo is symmetrically situated with its back turned 
towards the trophosphere. When the allantois appears and 
increases in size and the head enclosed in its “ pro-amnion ” 
1 R. Frommel, ‘Ueber die Entwickelung der Placenta von Myotus 
murinus,’ Wiesbaden, 1888, p. 27, 
