ril.iXdHS IX THE OITM. 



69 



likely that the initial changes are, in the majority of instances, accom- 

 plished in the tube. There, the ovum is bathed and moved about in 

 the clear fluid containing the spermatozoa, by the peristaltic action 

 of the walls of the duct, in order to enable the largest number of the 

 fertilising particles to obtain access to the yolk, and thus ensure 

 fecundity. 



With the formation of the embryo-cell, the vitellus becomes separated 

 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. 38). 



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

 (Fig. 39), 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. 40), and these again into other binary divisions 

 (Fig. 41) 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 

 impregnated parent embryo-cell. Each of these corpuscles contains a 

 colourless pellucid nucleus, and each of these again a nucleolus. The 



Fig. 38. 

 Ovum kkom OvmrrT nr Rabbit, 



PKNETKATKD HY Si'KRMATOZOA IN 

 ITS VlTKLLINK LaVKR. 



Fig. 39. 



An Ovum more auvan<ei) in 

 THB OvinufT. 



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-mucous substance 

 had been deposited thereon ; in the Rabbit the ovum acquires a thick 

 adventitious layer of albumin before entering the uterus (Fig. 42, n) ; in 

 the Guinea-pig the zona continues smooth. After entering the uterus, 

 on the fourth day the zona becomes fainter, as the final segmentation 

 or rnulben-y 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 viulbcrri/ 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 mulbeny-like body 



