PROCESS OF FERTILIZATION. /I 



indirect division are here entirely absent. The nucleus merely con- 

 tracts at a certain point and separates into two or more fragments 

 (direct fragmentation, Arnold) ; often the nucleus first assumes an 

 annular form and then breaks up into several fragments, which 

 remain loosely connected (polynuclear cells). Centrospheres are 

 also present, and appear to take a prominent part in the whole pro- 

 cess, although the exact relationship between the achromatin and 

 chromatin has not as yet been determined. 



Xemiloff has recently called attention to two locations where 

 amitotic divisions may readily be observed namely, in the large 

 surface cells of transitional epithelium of the bladder of mammals 

 and in the lymphoid tissue layer of the liver of amphibia. In the 

 cells of the former type the nuclear division is initiated by a division 

 of the nucleolus which is followed by a division of nucleus and later 

 the protoplasm. Centrosomes and attraction spheres were not 

 noticed in these cells. The division of the lymphoid cells of the 

 amphibian liver is initiated by a depression found in one side of 

 their spherical nuclei. This depression deepens until the nuclei be- 

 come perforated and assume an annular shape. These ring-shaped 

 nuclei then break through in two or more places and two or more 

 daughter nuclei are formed. During the process of division a cen- 

 trosome with attraction sphere may often be observed, generally 

 situated in the depression which initiates the division and later in 

 the center of the perforated nucleus. Its role in the division of the 

 nucleus and the .cell-body is, however, not fully understood. 



D. PROCESS OF FERTILIZATION. 



The sexual cells form a special group among cells in general. 

 Before the division of the egg-cell leading to the development of 

 the embryo can take place, the ovum must be impregnated (the so- 

 called parthenogenetic ova are an exception to this rule). Fertili- 

 zation is produced by the male sexual cell, the spermatozoon. 



The process of fertilization consists in a conjugation of two sex- 

 ual cells, and in this process certain peculiarities in the behavior of 

 both cells must be mentioned. 



The cell forming the ovum and the one forming the spermato- 

 zoon must pass through certain stages before fertilization can be 

 accomplished. These consist in the loss of half their chromosomes 

 by the nuclei of both sexual cells. In this way are produced the 

 matured sexual cells (ova and spermatozoa), which retain only 

 half of the number of chromosomes of a somatic (body-) cell. 

 In the conjugation of the male and female sexual cells their nuclei 

 unite to form a single nucleus, known as the segmentation nucleus. 

 Consequently, this nucleus contains the same number of chromo- 

 somes as does that of a somatic cell. 



In its earlier developmental stages the ovum is an indifferent cell, 

 the nucleus of which is known as the germinal vesicle. As the 



