354 SENESCENCE AND REJUVENESCENCE 



question whether the maturation divisions or either one of them 

 are in any way fundamentally different from other nuclear divi- 

 sions. They are beheved by many to be of great importance in 

 heredity, but until the problem of their cytological character is 

 solved any consideration of their significance for heredity must 

 remain in the field of speculation. 



The question of the physiological significance of maturation has 

 attracted little attention, but as a matter of fact it is in the answer 

 to this question that we shall find the key for the solution of the 

 other problems which have arisen in connection with maturation. 

 At least one of the maturation divisions, the so-called heterotypic 

 division — but whether the first or the second, opinions differ — has 

 commonly been supposed to be distinguished from ordinary divi- 

 sions by the behavior of the chromosomes, and much has been made 

 in a theoretical way of this difference. But with the extension of 

 our knowledge, one feature after another which was believed to be 

 characteristic of the maturation division has been found in other 

 divisions which have nothing to do with the development of the 

 gametes. The peculiar behavior of the chromatin, consisting in 

 premature division and agglutination of chromosomes to form rings 

 or other figures, which has been regarded as a characteristic feature 

 of the so-called "heterotypic" maturation division, has been 

 observed by Hacker, Bonnevie, and others in cleavage stages of 

 various forms, has also been found in the cells of mahgnant tumors, 

 and has been experimentally induced by the use of ether and chloro- 

 form and as a result of injury to the parent body.' Hacker is 

 inchned to believe that this heterotypic behavior of the chro- 

 mosomes indicates a low degree of differentiation, hence its occur- 

 rence in gametic history, in early cleavage, and in cancer cells 

 which are often regarded as a product of dedifferentiation. Hacker 

 was led to this conclusion by his behef , based on theoretical grounds, 

 that the gametes are undifferentiated cells containing germ plasm, 

 but from a physiological point of view both the stages in gametic 

 history where maturation occurs and the early cleavage stages are 

 stages of relatively high dift'erentiation. 



I Bonnevie, '08; Farmer, Moore, and Walker, '04; Hacker, '00, '04, '07; 

 Schiller, '09. 



