SEGMEN'TATION OF THE OVUM. 



59 



inner mass, i.m, and the subzonal layer, but at one side the inner mass remains 

 adherent to and closely connected with the subzonal layer. We now have 

 reached the stage in which the developing ovum may be designated as the blasto- 

 dermic vesicle. 



As to the interpretation of the parts, it is probable that the subzonal layer 

 is the ectoderm, and that the inner mass is the entoderm. At the stage we have 

 now reached the blastodermic vesicle has a large part of its walls formed by the 

 subzonal layer only, so that we call this the stage of the one-layered blastodermic 

 vesicle. 



Arrival in the Uterus. — During the stages described the ovum travels along 

 the Fallopian tube and reaches the uterus in an early phase of the stage which we 



im. 



km. 



Fig. 15. — Rabkit's Ovum of about Seventy 



Hours. 

 Ec, Outer layer, i.m. Inner mass of cells. Z, Zona 

 pellucida. — [Afler E. van Beneden.) 



Fig. 16. — Young Blastodermic Vesicle of a 



Mole. 

 i.Jit, Inner mass of cells, s.z. Outer or subzonal 

 layer, s, Zona pellucida. — [After W. Heape.) 



designate as the blastodermic vesicle. The transit requires about eighty hours in 

 the mouse, about five days in the opossum, four days in the rabbit, and eight to 

 ten days in the dog. The time necessary in man is unknown. It may be sup- 

 posed to be about one week. 



Pro-chorion. — The ovum in many mammals becomes surrounded by a gelat- 

 inous covering, which is secreted by the glands of the uterus. It may be com- 

 pared with the white of the bird's &gg. In the rabbit this envelope becomes 

 enormously thick about the blastodermic vesicle and in other rodents is volu- 

 minous. In the dog it is less developed, but presents the further peculiarity 

 that the secretion in the tubular glands may be hardened in connection with the 



