54 THE EARLY DEVELOPMENT OF MAMMALS. 



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 chromosomes are developed. When, as in the mouse, the chromosomes ap- 

 pear 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 seg- 

 mentation and in all subsequent cell divisions of this animal. It is believed to 

 be a general law that the male and female pro r nuclei each contribute the same 

 number of chromosomes to the segmentation spindle. This number is identical 

 with the number which appears during the reduction divisions which lead to the 

 maturation of the ovum, on the one hand, and the development of the sperma- 

 tozoon on the other; and, further, the number is one-half the number of chromo- 

 somes 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 succes- 

 sion 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.* 



Segmentation of the Ovum. 



Shortly after the entrance of the spermatozoon into the ovum the segmenta- 

 tion spindle is developed by the union of the pro-nuclei, as described in the pre- 

 vious section. This spindle leads to a division of the ovum into two cells. These 

 cells further rapidly divide. As stated on page 26, these early cell divisions are 

 called the segmentation of the ovum. 



The position of the segmentation spindle is always eccentric, and appears to 

 be approximately, if not exactly, the same as that of the egg-cell nucleus before 

 maturation. The axis of the spindle varies greatly in its direction. It is some- 

 times at right angles to the radius of the ovum, which passes through the polar 

 globules, but it is more usually oblique to this radius. It was at one time thought 

 that the plane of division was always at right angles to the radius of the extrusion 

 of the polar globules, but this view cannot be upheld. After the ovum has 



* " Lehrbuch der Entwicklungsgeschichte," sixth edition, 1898. The large Ascaris is a particularly 

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

 this form. Material suitably preserved may be obtained from the Preparator of the Department of Zoology, 

 University of Pennsylvania. 



