238 CONTINUITY 01 THE GERM-PLASM AS THE [IV. 



segments, would be able to produce the first division and 

 perhaps also the second and third, or some later division, but 

 that at a certain point during ontogeny, the nuclear substance 

 would become insufficient, and development would be arrested. 

 This will occur in eggs which enter upon development without 

 fertilization, but are arrested before its completion. One might 

 compare this retardation leading to the final arrest of develop- 

 ment, to a railway train which is intended to meet a number of 

 other trains at various junctions, and which can only travel 

 slowly because of some defect in the engine. It will be a little 

 behind time at the first junction, but it may just catch the train, 

 and it may also catch the second or even the third ; but it will 

 be later at each successive junction, and will finally arrive too 

 late for a certain train ; and after that it will miss all the trains 

 at the remaining junctions. The nuclear substance grows con- 

 tinuously during development, but the rate at which it increases 

 depends upon the nutritive conditions together with its initial 

 quantity. The nutritive changes during the development of 

 an egg depend upon the quantity of the cell-body which was 

 present at the outset, and which cannot be increased. If the 

 quantity of the nuclear substance is rather too small at the 

 beginning, it will become more and more insufficient in suc- 

 ceeding stages, as its growth becomes less vigorous, and differs 

 more from the standard it would have reached if the original 

 quantity had been normal. Consequently it will gradually fall 

 more and more short of the normal quantity, like the train 

 which arrives later and later at each successive junction, be- 

 cause its engine, although with the full pressure of steam, is 

 unable to attain the normal speed. 



It will be objected that four loops cannot be necessary for 

 nuclear division in Ascaris, since such division takes place in 

 the formation of the polar bodies, resulting in the appearance 

 of the female pronucleus with only two loops. But this fact 

 only shows that the quantity of nuclear substance necessary 

 for the formation of four loops is not necessary for all nuclear 

 divisions ; it does not disprove the assumption that such a 

 quantity is required for the division of the segmentation 

 nucleus. In addition to these considerations we must not 

 leave the substance of the cell-body altogether out of account, 

 for, although it is not the bearer of the tendencies of heredity. 



