FOUNDATION OF A THEORY OF HEREDITY. 175 



depends upon the fusion of two nuclei ; but the possibility of the 

 co-operation of the substance of the two germ-cells could not be 

 excluded, for in all the observed cases the sperm-cell was very small 

 and had the form of a spermatozoon, so that the amount of its cell- 

 body, if there is any, coalescing with the female cell, could not be 

 distinctly seen, nor was it possible to determine the manner in 

 which this coalescence took place. Furthermore, it was for some 

 time very doubtful whether the spermatozoon really contained 

 true nuclear substance, and even in 1879 Fol was forced to the con- 

 clusion that these bodies consist of cell-substance alone. In the 

 following year my account of the sperm-cells of Daphnidae followed, 

 and this should have removed every doubt as to the cellular nature 

 of the sperm-cells and as to their possession of an entirely normal 

 nucleus, if only the authorities upon the subject had paid more 

 attention to these statements 1 . In the same year (1880) Balfour 

 summed up the facts in the following manner ' The act of impreg- 

 nation 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 2 .' It is true that Calberla had already 

 observed in Petromyzon, 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 never- 

 theless be amply sufficient to transmit the tendencies of heredity. 

 Nageli and Pfliiger rightly asserted, at a later date, that the 

 amount of the substance which forms the basis of heredity is neces- 

 sarily 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 



1 Kolliker formerly stated, and has again repeated in his most recent publication, 

 that the spermatozoa (' Samenfaden') are mere nuclei. At the same time he re- 

 cognizes the existence of sperm-cells in certain species. But proofs of the former 

 assertion ought to be much stronger in order to be sufficient to support so improbable 

 a hypothesis as that the elements of fertilization may possess a varying morpho- 

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



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



