AUGUST WEI S MANN 349 



not lake place in all the eggs laid by an unfertilized female, but only in 

 part, and generally a small fraction of the whole, while the rest die. 

 But among the latter there are some which enter upon embryonic 

 development without being able to complete it, and the stage at which 

 development may cease also varies. It is also known that the eggs 

 of higher animals may pass through the first stages of segmentation 

 without having been fertilized. This was shown to be the case in 

 the egg of the frog by Leuckart, in that of the fowl by Oellacher, and 

 even in the egg of mammals by Hensen. 



Hence in such cases it is not the impulse to development, but the 

 power to complete it, which is absent. We know that force is al- 

 ways bound up with matter, and it seems to me that such instances 

 are best explained by the supposition that too small an amount of 

 that form of matter is present, which, by its controlling agency, effects 

 the building up of the embryo by the transformation of mere nutritive 

 material. This substance is the germ-plasm of the segmentation 

 nucleus, and I have assumed above that it is altered in the course of 

 ontogeny by changes which arise from within, so that when sufficient 

 nourishment is afforded by the cell-body, each succeeding stage neces- 

 sarily results from the preceding one. I believe that changes arise 

 in the constitution of the nucleoplasm at each cell-division which takes 

 place during the building up of the embryo, changes which either cor- 

 respond or differ in the two halves of each nucleus. If, for the pres- 

 ent, we neglect the minute amount of unchanged germ-plasm which 

 is reserved for the formation of the germ-cells, it is clear that a great 

 many different stages in the development of somatic nucleoplasm are 

 thus formed, which may be denominated as stages i, 2, 3, 4, etc., up 

 to n. In each of these stages the cells differ more as development 

 proceeds, and as the number by which the stage is denominated be- 

 comes higher. Thus, for instance, the two first segmentation spheres 

 would represent the first stage of somatic nucleoplasm, a stage which 

 may be considered as but slightly different in its molecular structure 

 from the nucleoplasm of the segmentation nucleus ; the first four 

 segmentation spheres would represent the second stage ; the succeed- 

 ing eight spheres the third, and so on. It is clear that at each suc- 

 cessive stage the molecular structure of the nucleoplasm must be 

 further removed from that of the germ-plasm, and that, at the same 

 time, the cells of each successive stage must also diverge more widely 



