20 HISTOLOGY 



the chromosomes in man. He found only two cells in which a count 

 could be made, in neither case with absolute certainty. But he agrees 

 with Duesberg that the reduced number is twelve, according to which 

 the whole number should be twenty-four. Recently, however, Wieman 

 has found cells in the brain of a 9-mm. human embryo which contained 33 

 chromosomes. Some cells in the nasal epithelium and mesenchyma of 

 this specimen contained 34, and others 38. Thus Wieman concludes 

 that the number in man is certainly greater than 24 and is perhaps vari- 

 able (Amer. Journ. Anat., 1913, vol. 14, pp. 416-471). 



In the grasshoppers, which are among the most favorable objects for 

 the study of mitosis, not only is the number of chromosomes for a given 

 species believed to be constant, but each cell appears to contain a definite 

 series of chromosomes, the members of which vary somewhat in shape 

 and size. Recent studies of such cells favor Rabl's hypothesis of the 

 individuality oj the chromosomes, according to which the chromosomes 

 persist in the resting nucleus, although disguised by their lateral branches 

 and diffuse granular form. If this hypothesis is correct, when a nucleus 

 prepares for division the same chromosomes which entered it will reappear. 

 Sometimes in the prophase the bands of chromatin are arranged hi a 

 polar field such as is seen in the telophase (Fig. 12, H). This arrange- 

 ment has been observed by Farmer and Shove in the prophase of Trades- 

 cantia, and by others in various plants and animals. It is regarded as 

 evidence that the chromosomes are "independent and continuously 

 perpetuated organs of the cell." Nevertheless it is generally true that 

 in resting nuclei no trace of individual chromosomes can be made out. 

 The great importance of accurate knowledge of the chromosomes is shown 

 by the following considerations. 



As a result of mitotic cell division, it is evident that every new cell 

 regularly receives one-half of each chromosome found in the parent cell, and 

 thus the number of chromosomes remains constant. But in the germ 

 cells the number is invariably reduced, and hi some animals it becomes 

 exactly one-half of the number found elsewhere in the body. In such a 

 case, when the male sexual cell, or spermatozoon, unites with the female 

 sexual cell, or mature ovum, in the process of fertilization, the original 

 number is restored. Each parent thus contributes one-half of the chromo- 

 somes found in the cell which gives rise to a new individual; and since 

 each of these divides with every subsequent cell division, it is evident 

 that one-half of the chromatin in every cell of the adult body is of mater- 

 nal origin and one-half of paternal origin. The process by which the 

 sexual cells acquire the reduced number of chromosomes and become 

 ready for fertilization is known as maturation. The production of the 

 sexual cells in the male is called spermatogenesis and in the female 

 ob'genesis. 



