1887] Embryology. 395 
Fleischmann has not yet been able, in spite of great care and 
patience, to find the ova of the cat and dog in process of segmen- 
tation in the oviducts. The youngest ovum which he found was 
a somewhat oval blastosphere, upon which the germinal area was 
already very distinct. This was invested by a very distinct Rau- 
ber’s layer of cells. 
The youngest blastosphere of the cat was nearly spherical, and 
twelve days after the first copulation still presents the form of an 
oblong sphere. Through rapid growth at the poles it soon, how- 
ever, becomes citron-shaped ; the germinal area then forms a con- 
vex elevation on the middle third of the blastosphere. 
While the blastosphere of the dog retains the two-pointed, 
citron-shaped form, that of the cat retains that form for only a 
very short time, and gradually becomes barrel-shaped, in that the 
points of the blastosphere are pressed inward by mutual pressure 
in the successive sections of the uterine cornua, so that the ends 
of the growing se gs. ean are only feebly conical. The flat- 
tened extremities of the blastosphere are not undergrown by 
of them. At the outer margins of the flattened ends of the bar- 
rel-shaped ovum there is a delicate reticulum formed of elevations 
of the ectoderm, which has apparently arisen by pressure of the 
ends of e hollow ovum upon the folds of the uterine mucous 
membran 
Aod ‘the entire germinal area and at the opposite side of 
the blastosphere, on the twelfth day, there are already formed 
small projections and elevations of the ectoderm, which serve to 
attach the ovum to its nidus. Before the allantois has reached 
any considerable dimensions the subzonal membrane has thrust 
out villi in all directions, and into these grows the connective . 
tissue supporting the outer vascular layer of the allantoic sac. 
The primitive groove is formed in the germinal area at right 
angles to the long axis of the blastosphere ; the same direction 
is assumed by the medullary groove. At about the sixteenth 
day the entire germinal area changes the direction of its axis to 
one parallel with that of the axis of the ovum, a condition which 
the embryo maintains until birth. 
-~ In the primitive streak the mesoderm is formed exclusively 
from the outer walls of the primitive groove; in many sections 
one sees the mesoderm proliferating outward from the sides of 
the primitive streak between the two primary embryonic layers, 
and numerous cleavage figures indicate rapid growth in this 
region. The entoderm is always distinctly marked off from the 
the medullary groove the mesoderm is always sharply marked 
off from the other layers; a heaping up of the mesoderm on the ` 
entoderm as described by E. van Beneden is not apparent. 
