714 



ANATOMY OF VERTEBRATES. 



Oviducnl ovum of Rabbit, pene- 

 trated by spermatozoa; iiia.srn. 

 350 diam. (confirmed in CCLXI".) 



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 

 ' niicropyle ' for the entry of the sperma- 

 tozoon has been detected in that delicate 

 evanescent tunic of the Mammalian ovum. 

 The l germinal vesicle,' or e 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. 



o 



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 ( 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- 



563 



Ovum, more advanced in tlie ovi- 

 duct, Rabbit; magi). 3JO diam. 



CCLXI". 



561 



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, 

 as in fig. 565 ; and so on until the whole is worked up into a 



1 CCLXI'', CCXLIX. for the same phenomena in Acephala (Unio and Anodon\ p. 526; 

 in Gastropods, p. 566. 



Ovum from tin- uuriiie half of the 

 oviduct, Ualiljii ; inagii. :\M diam. 



CCLXI". 



