396 SCIENCE PROGRESS 



rise to the tissues, epithelial, digestive, and muscular, of which 

 the body is built up. There finally remains only one blastomere, 

 whose nucleus does not undergo this degenerative change, and 

 this blastomere divides into two cells, which remain passive 

 during all the embryonic development, and even the youth, of 

 the worm, and then give rise to the genital organs. From 

 this development Weismann draws the conclusion that the 

 nuclear structure varies in different cells of the body, and that 

 only the genital cells retain the complete structure requisite 

 to form the whole body . 



Unfortunately, however, for Weismann, this case has been 

 critically examined by Boveri. It occasionally happens that 

 the eggs of Nematodes are entered by two spermatozoa, and then 

 abnormal development results which can exhibit several types. 

 From the comparative study of these types Boveri arrives at 

 the conclusion that the phenomenon of " diminution of the 

 chromatin," as the degenerative change is called, has nothing 

 to do with an unequal partition of nuclear material at a cell 

 division, but is the result of the particular type of cytoplasm 

 in which the nucleus is immersed. 



But in fact Weismann's objections are all bound up with his 

 idea of the scheme of development of the egg. He maintains 

 that in the nucleus of the fertilised egg there is hidden a sort of 

 miniature picture of the adult animal, and that development 

 consists in translating this plan into adult structure. " Onto- 

 geny, or the development of the individual, depends on a series 

 of graduated qualitative changes in the nuclear substance of the 

 egg-cell. " " The independently and hereditarily variable parts of 

 the body therefore serve as an exact measure for determining the 

 ultimate particles of which the germ-plasm is composed." He 

 accordingly conceives of the building up of the body of the 

 animal as the putting together of the pieces in a picture puzzle, 

 so that each part will only fit into the whole in a particular way. 



This conception has, however, received a number of rude 

 shocks in recent years. Even Weismann encountered enormous 

 difficulties when he descended to details. If the nucleus 

 of a cell in the developing rudiment of a leg, for instance, 

 contains only the potentialities which will allow to form 

 the tissues of the particular part of the leg in which it is 

 destined to be placed, how does it come about that, when the 

 leg of a newt is cut off, it can be regenerated ? Obviously these 

 nuclei must contain, besides their ordinary " plasm," some 

 " germ-plasm," and yet this germ-plasm cannot re-form a whole 

 animal, but only a leg. 



Then again, if the egg of a Frog or an Ascidian be allowed to 

 segment into four cells, and these four cells be separated along 

 the proper plane, or if the appropriate two be killed, the sur- 



