12 FERTILIZATION. 



plete in all its structure and functions. With the blending of the two pro-nuclei 

 the act of fertilization is completed, and the ovum is now capable of forming 

 new cells by division. Since the head of the spermatozoon is formed from the 

 nucleus of a seminal cell, part of which appears to be thrown off prior to the com- 

 plete maturation of the spermatozoon (Renson, Brown), and the female pro-nucleus is 

 the nucleus of an egg or germ-cell, part of which has been removed in the form of the 

 polar globules, the process of fertilization may be described as consisting essentially 

 of the conjunction of part of the nucleoplasm of a sperm cell with part of the nucleo- 

 plasm of a germ cell, the result being the production of a complete nucleus endowed 

 with active properties of division and reproduction. 



Although, as has been already stated, the changes which have just been described are most 

 clearly to be seen, and have been most completely studied, in the ova of echinoderms and 

 Ascaris, similar processes have been found to occur in most if not in all animals, and have 

 even been made out, although not very distinctly, in mammals (in the rabbit by v. Beneden). 

 There is no doubt, therefore, that the phenomena of fertilization are essentially the same 

 throughout the whole animal kingdom. As to the exact details of the process there is still 

 much discrepancy in the accounts given by recent observers. Of all those that given by 

 v. Beneden of the process of fertilization, and of the subsequent division of the resulting 

 nucleus in Ascaris. is the most explicit, and appears to negative the idea of a complete fusion 

 taking place between the elements of the pro-nuclei, at least so far as the chromatin is concerned. 

 According to this account (v. fig. 12), each of the two pronuclei is seen to possess, previous to their 

 conjunction, two short chromatin rods (chromosomes) imbedded in clear nuclear matrix. These 

 rods undergo various changes, resulting in the formation of a skein within each pro-nucleus 

 (II., III.), but eventually the skein resolves itself into two V-shaped loops or filaments (IV., V.). 

 On conjunction the matrix of the two nuclei may appear to blend, although it is doubtful if 

 they actually fuse together, but the chromatin filaments retain their distinct individuality. 

 The nucleus which is thus formed by the conjunction, contains, therefore, four similar V-shaped 

 chromatin filaments, which now split longitudinally (VI., VII.), and after being arranged for 

 a time at the equator of the now spindle-shaped nucleus (VIII.), four of the resulting filaments 

 pass towards the one pole, and form eventually the chromatin of the one daughter nucleus, 

 and four towards the other pole, eventually forming the chromatin of the other daughter 

 nucleus (XL. XII.). It is stated by v. Beneden that of each set of chromatin filaments, or 

 chromosomes, which thus separate from one another, one half the number is derived from the 

 male and the other from the female pro-nucleus. If this is the case, and if it should further 

 be shown that in every subsequent process of division of the resulting cells, the chromatin 

 filaments of the daughter cells are derived half from male chromatin filaments and half 

 from female, it necessarily follows that every cell nucleus must be regarded as containing 

 both male and female morphological elements. 



Meaning- of the polar globules. Theory of Minot. The question of the hermaphroditism 

 of cells was first raised by C. S. Minot in connection with the separation of the polar globules. 

 According to the view advocated by Minot, every cell which results from the division of a 

 fertilized ovum is hermaphrodite, for the fertilized ovum is formed by the union of both male 



Fig. 12. FORMATION AND CONJUGATION OF THE PRO-NUCLEI IN ASCARIS 



(E. v. Beneden.) 

 /, female pronucleus ; m, male pronucleus ; p, one of the polar globules. 



I. The second polar globule has just been extruded ; both female and male pronuclei contain two 

 chromatin particles ; those of the male pronucleus are becoming transformed into a skein. 



II. The chromatin in both pronuclei now forms a skein. 



Ila. The skein in the pronuclei is more distinct. Two attraction-spheres, each with a central 

 particle, united by a spindle of achromatic fibres, have made their appearance near the pronuclei. The 

 male pronucleus has the remains of the body of the spermatozoon adhering to it. 



III. The pronuclei are enlarged ; the skein formation of the chromatin is complete. 



