74 THE CELL. 



formed, which later arrange themselves in the form of chromo- 

 somes. 



After the second polar body has been extruded, the chro- 

 matin remaining in the ovum is transformed into the female pro- 

 nucleus. The latter then approaches the male pronucleus, the 

 membranes of both nuclei disappearing. The chromosomes of 

 the two nuclei thus formed are of equal number, and now come to 

 lie together. After a longitudinal division of the chromosomes, 

 the daughter chromosomes glide along the filaments of the achro- 

 matic spindle, developed from the centrosome of the male pronu- 

 cleus, toward its two poles, as in ordinary mitosis. This they do 

 in such a manner that an equal distribution of the male and female 

 daughter chromosomes results. Then follow the stages of the ana- 

 phase. 



From the above description of the process of fertilization it is 

 seen that it consists, in the end, of a union of the nuclei of both 

 sexual cells. 



If paternal qualities are inherited by the offspring, this can only 

 take place through the nucleus, or through the centrosome of the 

 male sexual cell. In other words, it can be safely said that these 

 structures, or the nucleus alone, are the principal means of trans- 

 mitting inherited qualities. The same may also be said of the 

 female pronucleus. There is no doubt that the first two seg- 

 mentation cells of the ovum are equally provided with male and 

 female nuclear elements. Since all future cells are derivatives of 

 these two, it is possible that the nucleus of every somatic cell 

 (body-cell) is hermaphroditic. 



E. CHROMATOLYSIS. 



In the living organism many cells are destroyed during the 

 various physiologic processes and replaced by new ones. On the 

 death of a cell, changes take place in its nucleus which result in its 

 gradual disappearance. These processes, which seem to follow 

 certain definite but as yet unfamiliar laws, have been known since 

 their study by Flemming (85, I) by the name of cliromatolysis 

 (karyolysis). The nuclei during the course of these changes show 

 many varied pictures. 



TECHNIC. 



In a fresh condition, cells do not show much of their internal 

 structure. Epithelial cells of the oral cavity, which can easily be ob- 

 tained and examined in the saliva, show really nothing except the cell 

 outlines and the nuclei. More, however, can be seen in young ova iso- 

 lated from the Graafian follicles of mammalia ; or the examination may be 

 facilitated by using the ovary of a young frog. Tissues that are especially 

 adapted for the observation of cells in a fresh condition are small ova, 

 blood-corpuscles, and epithelia of certain invertebrate animals (shellfish, 



