THE BLASTODERM 1C VESICLE. 



45 



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 

 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 from 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 gelatinous 

 covering, which is secreted by the glands of t the uterus. It may be compared 



km. 



FIG. 10. RABBIT'S OVUM or ABOUT SEVENTY 



HOURS. 



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

 pellucida. (After E. van Beneden.) 



S>z. 



FIG. ii. YOUNG BLASTODERMIC VESICLE OF A 



MOLE. 



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

 layer, z, Zona pellucida. (After W. Heape.) 



to the white of the bird's egg. In _the rabbit this envelope becomes enormously 

 thick about the blastodermic vesicle and in other rodents is voluminous. 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 envelope itself, which, 

 therefore, appears, when the ovum is removed from the uterus, to be studded over 

 with fine threads resembling villi. The gelatinous envelope has been termed by 

 Hensen the pro-chorion. The thread-like projections seen in the dog were taken 

 by Bischoff for true villi, and they have sometimes been referred to as the pro- 

 chorionic villi. The term pro-chorion has been applied to other structures, as, 

 for instance, to the subzonal layer of the blastodermic vesicle. The student needs 

 to be warned against confusing the term pro-chorion in its various applications. 



The Blastodermic Vesicle. 



The blastodermic vesicle always consists at first of the subzonal layer and an 



inner cell mass attached at one point to the subzonal layer, and has a cavity 



between the inner mass and the subzonal layer; the vesicle itself is always enclosed 



