1 889. J Embryology, 273 
pronounced manner traces of the primitive girdle-like placenta. 
It seems, in fact, as if that portion of the placental girdle 
directed away from the blood supply had been suppressed, 
leaving, as in the case of the red squirrel, only a segment of 
the original zonary placenta on the mesometric side. 
This diversity in the form of the placenta, even in types 
where the uterus is divided into a pair of tubular cornua, is 
associated with the mode of vascular supply of the uterine 
walls. In the cat, mouse and rabbit there is present a rich 
plexus of vessels all round the uterine tube interposed between 
the outer and inner muscular coats. The mouse has very few 
uterine glands, the rabbit and cat on the other hand have them 
very numerously imbedded in the wall of the mucosa. The 
area where active proliferation of the uterine wall goes on to- 
gether with hypertrophy of the uterine gland differs greatly in 
form in different types. In the mouse the hypertrophy is at 
first mainly confined to the connective tissues of the uterus ; in 
the rabbit, cat and squirrel it is at first mainly associated with 
changes in the size, form and thickness of the walls of the tubu- 
lar glands. All of these phenomena in turn are associated 
with the manner in which the blood supply for the maternal 
placenta is distributed. If the blood-Vascular supply is devel- 
oped mainly on the mesometric side, there appears to be a ten- 
dency to develop a discoidal placenta from the dorsal segment 
of the uterine mucosa which is in contact with the embryo and 
its membranes. If the blood-vascular supply of the uterine 
walls persists around the whole circumference of the tubular 
horn of the uterus there will be a tendency to develop a gir- 
dle-like placenta, as in the cat. If the muscular supply of the 
uterus opposite the mesometric side is, on the other hand, 
suppressed to any great degree, the continuation of the pla- 
centa fails to be formed on that side, and the quadrate seg- 
ment of the girdle leading finally to the discoidal form is de- 
veloped. As I have shown in a former note, that the mode of 
contact of the tubular uterine wall with the spherical ovum had 
something to do with the evolution of a zonary type of placen- 
tation, it may be well to indicate in this connection that there 
is also a physiological factor to be considered in the blood sup- 
ply of the uterus during gestation and the way in which such 
supply is modified. The factors at work in the differentiation 
of the placenta in the mammalia may be said to be mechano- 
physiological in character. The method of the establishment 
of formal relations between the surfaces of the embryo and 
parent during fcetal development are purely mechanical These 
