106 Cell Structure in Relation to Heredity 



stage in the division ; they only require to be arranged in the 

 nuclear plate and then distributed among the granddaughter-nuclei. 

 This method of division, which takes place with chromosomes already 

 split, and which have only to provide for the distribution of their 

 longitudinal halves to the next nuclear generation, has been called 

 homotypic nuclear division 1 . 



Reduction division and homotypic nuclear division are included 

 together under the term allotypic nuclear division and are dis- 

 tinguished from the ordinary or typical nuclear division. The 

 name Meiosis 2 has also been proposed for these two allotypic nuclear 

 divisions. The typical divisions are often spoken of as somatic. 



Observers who were actively engaged in this branch of recent 

 histological research soon noticed that the chromosomes of a given 

 organism are differentiated in definite numbers from the nuclear 

 network in the course of division. This is especially striking in the 

 gonotokonts, but it applies also to the somatic tissues. In the latter, 

 one usually finds twice as many chromosomes as in the gonotokonts. 

 Thus the conclusion was gradually reached that the doubling of 

 chromosomes, which necessarily accompanies fertilisation, is main- 

 tained in the product of fertilisation, to be again reduced to one half 

 in the gonotokonts at the stage of reduction-division. This enabled 

 us to form a conception as to the essence of true alternation of 

 generations, in which generations containing single and double 

 chromosomes alternate with one another. 



The single-chromosome generation, which I will call the haploid, 

 must have been the primitive generation in all organisms ; it might 

 also persist as the only generation. Every sexual differentiation 

 in organisms, which occurred in the course of phylogenetic develop- 

 ment, was followed by fertilisation and therefore by the creation of a 

 diploid or double-chromosome product. So long as the germina- 

 tion of the product of fertilisation, the zygote, began with a reducing 

 process, a special diploid generation was not represented. This, 

 however, appeared later as a product of the further evolution of the 

 zygote, and the reduction division was correspondingly postponed. 

 In animals, as in plants, the diploid generation attained the higher 

 development and gradually assumed the dominant position. The 

 haploid generation suffered a proportional reduction, until it finally 

 ceased to have an independent existence and became restricted 

 to the role of producing the sexual products within the body 

 of the diploid generation. Those who do not possess the necessary 

 special knowledge are unable to realise what remains of the first 



1 The name was proposed by W. Flernming in 1887; the nature of this type of 

 division was, however, not. explained until later. 



2 By J. Bretland Farmer and J. E. S. Moore in 1905. 



