THE PROCESS OF FERTILIZATION 297 



enlarged portion of the oviduct, within which a number of the 

 remarkable sperm-cells are always found in a mature female. They 

 are remarkable in being not thread-like, but rather spheroidal cells, 

 bearing, however, a small protuberance something like a pointed horn 

 (Fig- 75> ^. sp). When such a sperm-cell comes in contact with the 

 upper surface of an ovum a swelling forms at the place touched, and 

 the sperm-cell attaches itself firmly to this, and is drawn by it into the 

 ovum. Without doubt, amcsboid movements on the part of the sperm- 

 cell itself play some part in this, as can be most plainly seen in the 

 lai'ge sperm-cells of many Daphnids which we have already discussed. 

 In the egg of the thread-worm the whole sperm-cell with its nucleus 

 can soon be detected within the substance of the ovum, and it then 

 changes rapidly. Its main body fades more and more completely, 

 until at last it disappears altogether, while the nucleus becomes 

 vesicle-like and soon attains a considerable size (Fig. j^, B, spk). 

 Meanwhile the residue of the germinal vesicle which remained behind 

 in the ovum after the second directive division {B, Eik) has changed 

 into a large vesicle-like nucleus (C, ? k), which in the ovum of Ascaris, 

 as well as in the spermatozoon, at first contains a nuclear reticulum 

 with irregular fragments of chromatin. Later on, these form a spiral 

 coil in the manner we have already described, and finally this breaks 

 up into two large and relatively thick angular loops or chromosomes 

 (Fig- 75' G and D, chr). 



At the same time a nuclear division apparatus has formed in the 

 space between the two nuclei — the so-called male and female 'pro- 

 nuclei ' ( c? /c, ? k) — two centrospheres (csph) become visible, at first 

 lying close together, but afterwards moving apart (D) to form the 

 poles of a nuclear spindle, in the equatorial plane of which the four 

 chromosomes of the male and female pronuclei are now arranged. 

 The nuclear membranes disappear, and the two nuclei now unite to 

 form one, the segmentation nucleus (D). A dividing spindle then 

 develops and brings about the first embryonic cell-division (E), and 

 thus the beginning of the 'segmentation' of the ovum; each of the 

 four chromatin loops splits longitudinally, and each of the split halves 

 migrates, one to one, the other to the other daughter-nucleus {F). As 

 this same method of distribution of the chromatin substance is repeated 

 at every successive cell-division throughout embryogenesis, and indeed 

 through the whole of development, it follows that the result of 

 fertilization is, that all the cells of the body of the new animal which 

 develops from the ovum contain an equal quantity of paternal and 

 of maternal chromatin. If we are right in regarding the chromatin 

 substance as the hereditary substance, it becomes immediately apparent 



