FOUNDATION OF A THEORY OF HEREDITY. 233 



certain size and quantity in order to complete the whole ontogeny 

 (in a certain species), it follows that eggs possessing a reticulum 

 which contains barely enough nuclear substance to divide into 

 four 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 development, 

 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 continuously during de- 

 velopment, but the rate at which it increases depends upon the 

 nutritive conditions together with its initial quantity. The nu- 

 tritive 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 sub- 

 stance is rather too small at the beginning, it will become more and 

 more insufficient in succeeding 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, 

 because 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 assump- 

 tion that such a quantity is required for the division of the seg- 

 mentation nucleus. In addition to these considerations we must not 



