562 SUMMARY OF CUREENT RESEARCHES RELATING TO 



Tlie author has not yet been able, in spite of great care and patience, 

 to find the ova of the cat and dog in process of segmentation 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 in- 

 vested by a very distinct Eauber's layer of cells. 



The youngest blastosphere of the cat is 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, however, becomes citron- 

 shaped ; the germinal area then forms a convex 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 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 blastospheres are only feebly 

 conical. The flattened extremities of the blastosphere are not undergrown 

 by mesoderm, and therefore no vessels are developed in that portion of 

 them. At the outer margins of the flattened ends of the barrel-shaped 

 ovum, there is a delicate reticulum formed of elevations of the ectoderm, 

 which has apparently arisen by pressure of the ends of the hollow ovum 

 upon the folds of the uterine mucous membrane. 



Around 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 mesoderm, and the author could not obtain clear proof 

 of the entoblastic origin of the mesoderm. Even at the anterior end of 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. 



The mesoderm is characterized in well-preserved germinal areas, from 

 eleven to thirteen days old, as a solid mass of cells, which is composed of 

 several layers of cells under the germinal area, but consisting, outside of 

 the latter, of but a single layer of cells. 



The coelom first appears as clefts in the mesoblast outside of the 

 germinal area, and is pushed in under the latter at a later period. 



A chordal canal is always developed, and opens at a number of points 

 into the cavity of the umbilical vesicle or yelk-sac ; an opening of this 

 canal into the anterior end of the primitive streak was not discovered. 

 Only in an advanced embryo, with ten somites, could a slight ectodermal 

 dejiression be discovered at the anterior end of the primitive streak, but 

 this was closed below by a mass of cells. 



