210 ESSENTIALS OF BIOLOGY 



development of the fertilized ovum are of this equational kind and, 

 so far as the chromatic material of the nucleus is concerned, there 

 is no difference between them. 



So Weismann postulated another kind of nuclear division. 

 Let us suppose that the fertilized ovum contains the determinants, 

 fl, 6, c, d, e, /. When it divides these determinants are supposed 

 to become separated from each other during the nuclear divisions 

 so that while the original cell (say the ovum) contained determin- 

 ants of all the characters, a, b, c, d^ e, and /, the daughter-cells 

 would, each of them, contain only one determinant and would 

 be instrumental in the development of only one organ, tissue, 

 character, etc. Some time, then, during the segmentation of the 

 ovum the various determinants would be sorted out and the 

 embryo would become a mosaic of formative agencies, each of 

 these being located in a single cell, or in a small number of such. 

 These formative cells would then divide to form the organ- 

 anlagen in virtue of the determinants carried by them. 



This notion of determinants of structure was one of the essential 

 parts of Weismann 's hypothesis of development and it is both 

 curious and instructive that it should have been accepted in 

 spite of the want of any evidence that the segmentation cell 

 nuclei differed from each other. Another essential feature was 

 the notion that the formative agencies, or determinants, were 

 physical-chemical in their nature — this we shall consider later. 

 Now these two essential ideas : that of formative, particular 

 agencies in the chromatic material of the nuclei, and that of 

 the physical-chemical nature of the formative and particular 

 agencies are also included in current, genetical hypotheses. 



74Z>. " MoRGANiSM " : THE Genes. Thcsc current hypo- 

 theses hold that the chromosomes are linear series of discrete, 

 chromatin-particles that are^ or carry the formative agencies for, 

 the development of certain organic characters. During the pro- 

 cesses of maturation and fertilization these particles become 

 separated, joined, sorted and reassorted and it is held that there 

 is a correlation between the joinings, disjoinings, assortments, etc., 

 of the formative particles and certain groups of bodily characters 

 that develop in the organisms in which the joinings, disjoinings, 

 etc., occur. It is true that the nuclear phenomena mentioned are 

 not held to occur in the cell-divisions that occur after the fertiliza- 

 tion of the ovum : nevertheless, it will be seen that the hypotheses 



