CHANGES IN THE OVUM. 65 



or retracted by fluid from the zona pellucida, and begins to rotate 

 therein ; while one or two minute granular or oil-like bodies may appear 

 in the surrounding fluid (Fig. 28). 



A division or segmentation of this primary embryo-cell into two portions 

 (Fig. 29), each provided with a nucleus, is the next step ; then there is 

 mutual repulsion of these secondary globes, and further cleavage of each 

 into two portions (Fig. 30), and these again into other binary divisions 

 (Fig. 30) of the germ yolk, through attraction round each cell of the 

 particles contiguous thereto, until the whole is worked up into a mass of 

 finely 'nucleated corpuscles — the ultimate segmentations of the impreg- 

 nated parent embr3^o-cell. Each of these corpuscles contains a colorless 

 pellucid nucleus, and each of these again a nucleolus. The eight-fold 

 cleavage of the yolk has been observed three days after impregnation in 

 the Rabbit, four days in the Guinea-pig, and ten days in the Bitch ; and 

 always in the ova found towards the uterine extremity of the oviduct. In 

 the latter animal the smooth surface of the zona pellucida becomes 

 irregularly flocculent, as if a granulo-mucus substance had been deposited 



Ovum from the Uterine half of 

 Oviduct. 



Fig. 31- 



Ovum from the Uterine end of the 

 Oviduct, with an additional layer 

 OF Albumen. 



thereon ; in the Rabbit the ovum acquires a thick adventidous layer of 

 albumen before entering the uterus (Fig. 31, <7) ; in the Guinea-pig the 

 zona continues smooth. After entering the uterus, on the fourth day the 

 zona grows fainter, as the final segmentation or mulberry state of the 

 yolk is attained, and it disappears altogether when the germ-mass is 

 completed. The act of impregnation is thus consummated, and a series 

 of new changes begin, which are replete with interest and importance. 



When arrived in the uterus, a layer of very small vesicles makes its 

 appearance on the whole of the inner surface of the membrane now 

 investing the yolk. The mulberry structure then passes from the centre 

 to a certain part of that layer, the vessels of the latter coalescing with 

 those of the former, where the two sets are in contact, to form a mem- 

 brane — the future amnion ; while the interior of the mulberry-like body 

 is now seen to be occupied by a large vesicle, containing a fluid and 

 dark granules. In the centre of this fluid is a spherical body, com- 

 posed of a substance having a finely granular appearance, and containing 

 a cavity filled with a colorless and pellucid fluid ; this hollow and 



5 



