2o8 ESSENTIALS OF BIOLOGY 



preformation^ according to which the adult parts actually existed, 

 somehow, in the ovum or spermatozoon. The cruder, pre- 

 formationist notions sometimes expressed it rather naively, as, 

 for example, when the human spermatozoon was actually figured 

 as containing a homunculus. The well-developed preformist 

 notions, as for example, that formulated in Darwin's pangenesis, 

 were, of course, much more subtle, though they were incapable 

 of experimental verification. So were the later preformation 

 hypotheses of Weismann, and in our own time those of Morgan 

 and his colleagues and pupils. There did seem, however, to be 

 an actual basis of observational data in Weismann's speculations 

 and so also with the later work which resulted from the partially 

 abandoned Weismannism. Then came the *' Mendelian Renais- 

 sance " and the methods of modern genetics and, when it became 

 possible to make correlations between the results of breeding 

 experiments and morphological changes in the constituents of 

 the nuclei of the conjugating ova and spermatozoa, the modern 

 preformationism did seem to have both observational and experi- 

 mental basis. These latter notions of developmental processes 

 we must now discuss. 



74<a!. The Morphology of the Nucleus in Development. 

 Certain things made it probable that nuclear processes are 

 actually bound up with embryogenies and subsequent develop- 

 mental processes, (i) The complexities of chromatin-structure 

 opened out a new field for the application of the morphological 

 method on the microscopic scale ; (2) the behaviour of the 

 chromosomes both in the phenomena of maturation and fertiliza- 

 tion were very suggestive of the significance of these nuclear 

 structures. Such investigations led to the conceptions of Weis- 

 mannism. 



There appeared to be specific " outfits " of discrete chromo- 

 somes in all organisms that were minutely studied. Every cell 

 in the Soma in all the individuals of a species of organisms has 

 {when it divides) a certain number, A^, of chromosomes, and every 

 one of the matured ova and spermatozoa of the same individuals 

 has (when it is formed) a number, half A^, of chromosomes. The 

 reduction of A^ to half A^^ occurs in the processes of maturation. 

 The numbers A' and half A" are not actually constant, as will be 

 seen by a candid study of the results of individual investigations : 

 still there is much in the general statement given above that 



