500 



EVOLUTION, HEREDITY, EUGENICS 



arise from primordial cells. Early in embryonic development it is 

 difficult to tell whether these cells are to form male or female gonads. 

 Cell Division. — The multiplication of cells by the process of 

 cell division continues after maturity, and in most cases throughout 

 life. Cell division consists of direct division or amitosis and, most 

 commonly, of indirect division or mitosis. 



Direct Cell Division — Amitosis. — In this form the nucleus 

 divides into two daughter nuclei without any apparent preliminary 



change in its structure. 

 The division of the nucleus 

 may or may not be fol- 

 lowed by division of the 

 cell body. 



Amitosis is found in 

 the epithelium of the blad- 

 der, and in the placenta. 

 It is apparently normal 

 in, although not exclus- 

 ively characteristic of cart- 

 ilage, tendon and bone- 

 marrow. Some hold that 

 Fig. 258. Epithelial cells from ovary of it is a sign of degeneration, 

 cockroach. (From Bailey's Histology. Courtesy while Others believe it tO 

 of William Wood & Co.) be a sign of rejuvenation. 



In Protozoa (page 42) it 

 regularly occurs, as does mitosis also, but in the germ cells and 

 in early embryonic cells division is indirect, e.g. by mitosis. 



Indirect Cell Division — Mitosis. — In this form of cell division the 

 nucleus gives rise to two daughter nuclei, but only after the chro- 

 matin content has undergone certain characteristic changes. 

 /. Prophase: (a) As the cell prepares for division there is a 

 transformation of the nuclear substance involving both physical 

 and chemical changes. The chromatin loses its net-like arrange- 

 ment, increases in staining power and eventually forms a definite 

 number of chromosomes characteristic of the species. 



At first the chromatin is transformed into a convoluted, closely 

 coiled thread, the skein or spireme. In many species a longitudinal 

 split in the chromosomes becomes apparent at this time. The 

 closed skein gradually opens, the nuclear membrane disappears, and 

 the nucleoli are merged in the skein. In some forms the chromo- 



