IV.] FOUNDATION OF A THEORY OF HEREDITY. 23 1 



Hence when we see that the eggs of many animals are 

 capable of developing without fertilization, while in other 

 animals such development is impossible, the difference be- 

 tween the two kinds of eggs must rest upon something more 

 than the mode of transformation of the nucleus of the germ-cell 

 into the first segmentation nucleus. There are, indeed, facts 

 which distinctly point to the conclusion that the difference is 

 based upon quantitative and not qualitative relations. A large 

 number of insects are exceptionally reproduced by the par- 

 thenogenetic method, e.g. in Lepidoptera. Such development 

 does not take 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 

 ^^g of the frog by Leuckart ^, in that of the fowl by Oellacher ^, 

 and even in the ^gg 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 always 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 con- 

 trolling 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 as- 

 sumed 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 necessarily results from the preceding one. I believe 



^ R. Leuckart, — article ' Zeugung,' in R. Wagner's ' HandwOrtcrbuch 

 der Physiologic,' 1853, Bd. IV. p. 958. Similar observations were made 

 by Max Schultze. These observations appear however to be erroneous, 

 for Pfluger has since shown that the eggs of frogs never devclope if the 

 necessary precautions are taken to prevent the access of any spermatozoa 

 to the water.— A. W., 1888. 



2 Oellacher, ' Die Veranderungen des unbcfruchtetcn Keims des 

 Huhncheneies.' ' Zeitschrift fur wissenschaftliche Zoologie,' Bd. XXII. 

 p. 181. 1872. 



^ Hensen, ' Centralblatt,' 1869, No. 26. 



