IMPREGNATION OF THE OVUM. 41 



always to move toward a central position. The force which draws the pro-nuclei 

 together is unknown. The hypothesis that this force is chemotropic has met with 

 favor. 



The Fusion of the Pro-nuclei. In the rabbit, as probably in all mammals, both 

 pro-nuclei lie at first eccentrically, but both move toward each other and toward 

 the center, meeting when the central position is attained. As they near one an- 

 other both pro-nuclei perform active amoeboid movements. After they meet they 

 still continue their amoeboid movements, and travel together to the center of the 

 ovum (Fig. 5). One of the pro-nuclei assumes a crescent shape and embraces the 

 other. In the mouse the history is similar. After the two pro-nuclei in this animal 

 have met, they remain side by side, but they are without membranes. After the con- 

 junction of the pro-nuclei the chromatin threads become distinct and draw closer 

 together. Between them appears first a small spot or centrosome with a few radi- 

 ating lines around it (Fig. 117). From the centrosome arises a spindle of achro- 

 matic threads (Fig. 118). The chromosomes, both male and female, attach them- 

 selves to the spindle, and therewith impregnation is completed and mitosis of 

 the impregnated ovum initiated. 



It is now believed that the pro-nuclei never unite to form a distinct mem- 

 branate nucleus, the so-called segmentation nucleus of earlier writers, but that the 

 fusion always takes place during the absence of the membranes of the pro-nuclei by 

 the mingling of their contents. The time of mingling, however, varies as regards 

 the formation of the chromosomes. It may take place before or after the chromo- 

 somes are developed. When, as in the mouse, the chromosomes appear as two 

 distinct groups, it is possible sometimes to determine their number. In the mouse 

 counting is difficult, but there seems little doubt that each pro-nucleus forms 

 twelve chromosomes. Hence it results that there are twenty-four chromosomes in the 

 segmentation spindle. This number, twenty-four, so far as has been determined, is 

 the number which appears during later stages of segmentation and in all subse- 

 quent cell divisions of this animal. It is believed to be a general law that the male 

 and female pro-nuclei each contribute the same number of chromosomes to the seg- 

 mentation spindle except in those cases where an accessory chromosome is inter- 

 polated in the development. This number is identical with the number which ap- 

 pears during the reduction divisions which lead to the maturation of the ovum on 

 the one hand and the development of the spermatozoon on the other; and, 

 further, the number is one half the number of chromosomes which appear during 

 ordinary cell divisions of the species. The most thorough study of the phenomenon 

 which has yet been made is that by a succession of able investigators upon the 

 large nematode Ascaris megalocephala. An admirable summary of the process of 

 fertilization in Ascaris has been given by Oscar Hertwig.* 



* " Lehrbuch der Entwicklungsgeschichte," eighth edition, 1906. The large Ascaris is a particularly 

 favorable object. The student who wishes to pursue the practical study of impregnation further should select 

 this form. 



