466 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



rious manner. The work of Pfeffer, 1 who found that in the fertilization of 

 ferns malic acid within the female organs attracts the spermatozoids to their 

 vicinity, suggests strongly that also among animals the attraction may be a 

 chemical one, the ovum containing or producing something for which the sper- 

 matozoon has an affinity. If so, the meeting of the two germ-cells is an illus- 

 tration of a widespread principle of nature known as chemotaxis, or chemo- 

 tropism. Experimental evidence upon the subject in animals is wanting. 



Fertilization. — It will be remembered that the ovum and the spermatozoon 

 undergo in their growth the process of maturation, and that this process con- 

 sists essentially of a loss of one-half of the chromosomes of their nuclei. The 

 germ-cells thus matured meet, as we have seen, in the distal half of the Fal- 

 lopian tube and fuse into one cell, the process of fusion being called fertiUzation 

 or impregnation. The details of fertilization have not been observed in the 

 case of the human being, and the following account is generalized from our 

 knowledge of the process in other mammals and lower animals. In its broad 

 outlines fertilization is probably the same in all animals, the differences being 

 confined to details. 



The ovum at the time of fertilization is surrounded by the zona radi- 

 ata alone, the corona radiata having been lost. The spermatozoa swarm 

 about the zona, lashing their tails and attempting to worm their way through 

 it. Several may succeed in reaching the perivitelline space, but for some 

 unknown reason in most cases one ouly penetrates the substance of the ovum ; 

 the others ultimately perish. In mammalian ova there is no micropyle, and 

 apparently the successful spermatozoon may enter at any point, the protoplasm 

 of the egg rising up as a slight protuberance to meet it (Fig. 223, a). In some 

 animals the tail is left outside to perish ; in others it enters, but then disap- 

 pears; in no case does it appear to be of further use. The head and probably 

 the middle-piece are of vital importance. The head, now known as the sperm- 

 nucleus or male pronucleus, proceeds by an unknown method of locomotion 

 toward the centre of the egg, and becomes enlarged by the imbibition of liquid 

 (Fig. 223, b). The matured nucleus of the ovum, or egg-nucleus, also moves 

 slowly toward the future meeting-place of the two nuclei, which is near the 

 centre of the egg. The two finally meet (Fig. 224, c) and together form a 

 new and complete nucleus, called the first segmentation-nucleus (Fig. 224, i>). 



This body has the conventional nuclear structurt — namely, an achr atic 



network with the chromatic reticulum mingled with it — and the whole is 

 covered by a nuclear membrane. From the observations of Van Beneden, 

 Riickert, 2 Zoja, 3 and others, it seems to be a fact that the male and the female 

 chromosomes do not fuse together, but remain distinct from each other, per- 

 haps throughout all the tissue-cells. The chromosomes, it will be perceived, 



are now restored to the original nber presenl in either germ-cell before its 



maturation, hence in the human being perhaps sixteen, one-half of them 



1 \V. Pfeffer: Untersuchungen au&dem Botanischen Tn&titutzu Tubingen, 1884, i. 

 -J. Riickert: Archiv fiir mikroskopische Anatomic, 1895, xlv. 

 K. Zoja: Anatomischer Anzeiger, 1896, xi. 



