714 



ANATOMY OF VERTEBRATES. 



Ovlducal ovum of Rabbit, pene. 

 (rated by spermatozoa ; majrn. 

 350 diam. (confirmed in cclxi".) 



563 



the ovisac, prior to its expulsion, but they have never been 

 traced so far. They were first seen, by Martin Barry, to 

 have penetrated the ' zona pellucida,' in a Rabbit's oviducal ovum, 



fig. 562. No definite single pore or 

 6 micropyle ' for the entry of the sperma- 

 tozoon has been detected in that delicate 

 evanescent tunic of the Mammalian ovum. 

 The 'germinal vesicle,' or 'germ-cell,' 

 disappears as such. A somewhat more 

 opaque ' embryonal cell ' succeeds, which 

 may be, or includes, a combination of the 

 nuclear matter of the sperm-cell with 

 that of the germ-cell. Then follow 

 the initial steps, figs. 563-565, which 

 Barry's capital discovery showed to be the same essentially 

 in Mammals as in all lower animals ; and the entire yolk under- 

 goes the cleavage-process in its combina- 

 tion with the progeny of the embryonal 

 cell. Most of these initial steps are taken 

 in the course of the impregnated ovum 

 through the oviduct. 



While in this narrow tube the ova are 

 rolled to and fro by its peristaltic actions 

 in a transparent fluid more or less abound- 

 ing with spermatozoa; and the more of 

 these get access to the yolk the more 

 certain and complete is its segmentation. 

 With the formation of the embryo-cell 

 the yolk becomes separated by fluid from the f zona pellucida,' 

 and begins to rotate therein, as indicated by the arrows in fig. 

 562 ; one or two minute granular or oil- 

 like bodies may appear in the surround- 

 ing fluid. 1 



A division of the primary embryo-cell, 

 with mutual repulsion of the two second- 

 ary ones, is followed by cleavage of the 

 entire yolk, through attraction round each 

 secondary cell, fig. 563, of the parti- 

 cles contiguous thereto. A repetition of 

 this process issues in the four divisions of 

 the germ-yolk, fig. 564 ; then in the eight, 

 and so on until the whole is worked up into a 



Ovum, more advanced in the ovi- 

 duct, Rabbit; magu. 350 diam 

 cclxi". 



561 



Ovum from the uterine half of the 

 oviduct, Rabbit; magn. 350 diam. 

 cclxi". 



as in ficr. 



565 



1 cclxi", ccxlix. for the same phenomena in Acephala ( TJnio and Anodon), p. 

 in Gastropods, p. 566. 



526; 



