Proceedings of the Association of American Anatomists XI 
The blastocyst passes into the fixation chamber, the trophoblast devel- 
ops at the anti-embryonal pole a syncytial enlargement, the fixation mass 
which perforates the uterine epithelium and spreads out between the 
surrounding epithelium and the mucosa; this mass soon develops root- 
like processes which project into the vascular mucosa. This condition 
continues while the blastocyst is increasing in size and forming a de- 
cidual cavity at the expense of the intermediate portion; as this cavity 
is developed, the lateral trophoblast of the blastocyst causes a destruction 
of the lining epithelium; finally a zone of trophoblast surrounding the 
germinal area is brought in contact with the margin of the placental 
chamber, and thus is begun the formation of the true placenta. As 
this stage is reached, the blastocyst becomes entirely separated from 
the uterine mucosa of the fixation chamber by degeneration of the root- 
like processes of the fixation mass. The trophoblastic covering of the 
germinal area (Rauber’s iayer) has now disappeared and then follows a 
folding over of the marginal trophoblast and borders of the germinal 
area to form the amniotic cavity; the united trophoblast (serosa) extends 
into the placental chamber and causes destruction of its lining epithe- 
lium which up to this time has been unaltered. Where the uterine 
mucosa comes in contact with the trophoblast there are developed vas- 
cular channels which penetrate the trophoblast, and the allantoic out- 
growth of the embryo has spread out as a disk on the underside of 
the trophoblast and gives rise to vessels which penetrate it. 
Fiber zibethicus shows in its early development the so-called inver- 
sion of layers and presents a number of interesting facts in comparison 
with the other described Muridae. 
A detailed description of the preplacental stages of Spermophilus 
is now in press, and a paper now in preparation will figure and describe 
the formation and structure of the true placenta of Spermophilus, Fiber 
and other hitherto undescribed rodents. 
NOTES ON THE VARIATION IN ORIGIN OF THE INTERNAL CAROTID 
OF THE HORSE. By Grant S. Hopkins. Department of Veterinary 
and Comparative Anatomy, Cornell University. 
The origin of the internal carotid artery was noted in fifteen cases—on 
both sides in eight of them, on one side only in the remainder. In eight 
cases the internal carotid originated as commonly described, namely, as 
one of the terminal branches of the common carotid. In the remaining 
seven, the relation of the internal carotid to the occipital and external 
carotid varied considerably. In three specimens it originated from one 
to eight centimeters posterior to the occipital; in three others, it and 
20 
