178 CONTINUITY OF THE GERM-PLASM AS THE [IV. 



up the facts in the following manner : * The act of impregnation 

 may be described as the fusion of the ovum and spermatozoon, 

 and the most important feature in this act appears to be the 

 fusion of a male and female nucleus ^' It is true that Calberla 

 had already observed in Petroiuyzon, that the tail of the 

 spermatozoon does not penetrate into the egg^ but remains 

 in the micropyle ; but on the other hand the head and part 

 of the 'middle-piece' which effect fertilization, certainly contain 

 a small fraction of the cell-body in addition to the nuclear 

 substance, and although the amount of the former which thus 

 enters the egg must be very small, it might nevertheless be 

 amply sufficient to transmit the tendencies of heredity. Nageli 

 and Pfluger rightly asserted, at a later date, that the amount 

 of the substance which forms the basis of heredity is necessarily 

 very small, for the fact that hereditary tendencies are as strong 

 on the paternal as on the maternal side, forces us to assume 

 that the amount of this substance is nearly equal in both male 

 and female germ-cells. Although I had not published anything 

 upon the point, I was myself inclined to ascribe considerable 

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

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

 investigations upon DapJuiidae had shown that an animal pro- 

 duces large sperm-cells with an immense cell-body whenever 

 the economy of its organism permits. All Daphnidac in which 

 internal fertilization takes place (in which the sperm-cells are 

 directly discharged upon the unfertilized egg), produce a 

 small number of such large sperm-cells {Sida, Polvp/ionus, 

 Bythotrephes) ; while all species with external fertilization 

 {Dapluiidae, Lynceiiiae) 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, 



the elements of fertilization may possess a varying morphological value. 

 Compare Zeitschr. f. wiss. Zool., Bd. XLII. 



* F. M. Balfour, ' Comparative Embryology,' vol. i. p. 69. 



