318 



THE CAT. 



[CHAP. x. 



the eighth chapter,* with its yelk, zona pellucida, germinal vesicle 

 and spot, the first change preparatory to impregnation is the rupture 

 of the Graafian follicle. Having been cast forth from that follicle 

 and entered the Fallopian tube, the ovum spontaneously begins the 

 developmental process itself. Its nucleus, or germinal vesicle, ap- 

 proaches the surface of the ovum, and its outline becomes irregular or 

 obscure. Having reached the surface, one or two portions of its sub- 

 stance, called polar vesicles, are extruded through it from the ovum. 

 The remnant of the germinal vesicle then returns towards the centre 

 of the ovum, and constitutes the central, or so-called "female " pro- 

 nucleus. It probably contains a nucleolus, and the protoplasm of the 

 ovum becomes radiately striated around it. 



All is ready now for the act of impregnation. 



3. The spermatozoa having advanced to the ovum (for by their 

 vibratile action they can make their way up the uterine cornua and 

 Fallopian tubes), no sooner meet with it, than they plunge into 

 its outer transparent coat, and one spermatozoon (possibly,f but not 

 probably, more than one) actually bores its way through, and plunges 

 its head into the substance of the ovum. That substance itself, how- 

 ever, is not altogether passive, for a small prominence may arise 

 from its surface to meet the incoming spermatozoon. Immediately 

 this contact has taken place, the outermost layer of the ovum's pro- 

 toplasm may separate itself off as a distinct membrane, and so bar 

 the way to the entrance of other spermatozoa. 



The head of the spermatozoon having thus entered the ovum, it 

 constitutes there a second peripheral or "male" pronucleus. This 

 advances towards the centre of the ovurn, the protoplasm of which 

 assumes a radiate arrangement round it. Then the two pronuclei 

 are attracted towards each other and meet and fuse into a single 

 body called the "first segmentation nucleus (Fig. 141, A). This 

 nucleus has thus been formed by two corresponding parts from 

 the male and female organisms respectively. For the female 

 pronucleus is a portion of the nucleus of a germ cell, the whole 

 of which has become modified into the ovum, while the male 

 pronucleus is a spermatozoon, which is a part of the nucleus of 

 the original sperm-cell, which divided to form spermatozoa. "We 

 have therefore in the first segmentation nucleus, what must rank 

 as a true nucleus of complex nature and diverse origin an origin 

 from two equivalent and complementary parts. 



4. The next step in the process of development is that known as the 

 SEGMENTATION or THE YELK, or vitellus, /. c., of the protoplasm of the 

 ovum. The first segmentation nucleus gradually elongates, becomes 

 medianly constricted, and divides into two secondary segmentation 



* See ante, p. 250. 



t There is as yet much divergence of 

 opinion on this matter, and some ob- 

 servers even maintain that the entrance 

 of more than one spermatozoon is an 

 occasion of monstrosity in the embryo. 



The reader will find a full statement and 

 discussion of these views in Mr. F. 

 Balfour's excellent Treatise on Com- 

 parative Embryology, vol. i., 1880, 

 p. 67. 



