176 THE CONTINUITY OP THE GERM-PLASM AS THE 



importance to the cell-substance in the process of fertilization ; 

 and I had been especially led to adopt this view because my 

 investigations upon Dapkniclae had shown that an animal produces 

 large sperm-cells with an immense cell-body whenever the economy 

 of its organism permits. All Daplmidae in which internal fertiliza- 

 tion takes place (in which the sperm-cells are directly discharged 

 upon the unfertilized egg), produce a small number of such large 

 sperm-cells (Sida, Polyphemus^ Bytkotrepkes] ; while all species 

 with external fertilization (Daphnidae, Lynceinae) produce very 

 small sperm-cells in enormous numbers, thus making up for the 

 immense chances against any single cell being able to reach an 

 egg. Hence the smaller the chances of any single sperm-cell 

 being successful, the larger is the number of such cells produced, 

 and a direct result of this increase in number is a diminution 

 in size. But why should the sperm-cells remain or become so 

 large in the species in which fertilization is internal? The idea 

 suggests itself that the species in this way gains some advantage, 

 which must be given up in the other cases ; although such ad- 

 vantage might consist in assisting the development of the fertilized 

 ovum and not in any increase of the true fertilizing substance. At 

 the present time we are indeed disposed to recognize this advantage 

 in still more unimportant matters, but at that time the ascertained 

 facts did not justify us in the assertion that fertilization is a mere 

 fusion of nuclei, and M. Nussbaum 1 quite correctly expressed the 

 state of our knowledge when he said that the act of fertilization 

 consisted in ' the union of identical parts of two homologous cells.' 

 Pfluger's discovery of the 'isotropism' of the ovum was the 

 first fact which distinctly pointed to the conclusion that the bodies 

 of the germ-cells have no share in the transmission of hereditary 

 tendencies. He showed that segmentation can be started in 

 different parts of the body of the egg, if the latter be permanently 

 removed from its natural position. This discovery constituted an 

 important proof that the body of the egg consists of a uniform 

 substance, and that certain parts or organs of the embryo cannot 

 be potentially contained in certain parts of the egg, so that they 

 can only arise from these respective parts and from no others. 

 Pfliiger was mistaken in the further interpretation, from which he 

 concluded that the fertilized ovum has no essential relation to the 



1 Arch. f. mikr. Anat., Bd. 23. p. 182, 1884. 



