Lefevre — The Advance of Zoology in the Nineteenth Century. 99 



malcule living in the sperm or seminal fluid of the male 

 (hence the name spermatozoon) was in 1831 proven by 

 Kolliker to arise directly from cells of the testis, and some- 

 what later its precise cellular nature was demonstrated. The 

 spermatozoon, like the egg, is therefore a true cell, though 

 considerably modified for its special function. 



A most important step for the correct understanding of 

 physical inheritance was next taken when Oscar Hertwig in 

 1875 showed that fertilization is accomplished by the union 

 of the nucleus of one spermatozoon, which penetrates the 

 egg, with the nucleus of the egg. During the past twenty- 

 five years the exact details of this process have been made 

 known through the brilliant researches of later cytologists. 

 In fertilization we now know that the male and female nucleus 

 contribute to the formation of the nucleus of the fertilized 

 egg exactly equivalent amounts of chromatin which, during 

 cleavage, is therefore distributed equally to the daughter-cells 

 arising at each division. The marvelous result is that the 

 determining constituent, or chromatin, of every cell of the 

 body is derived half from one parent and half from the other. 

 In this fact rests the physical basis of inheritance, an in- 

 heritance which is therefore twofold and transmits to the 

 offspring the characteristic organization of both parents. 

 Within recent years we have witnessed the establishment of 

 the all-important fact that chromatin is identical with the 

 germ-plasm, the substance which contains the sum- total of 

 the species and which, when detached from the parent body, 

 under the proper conditions gives rise to the body of the 

 child. The problem of heredity, therefore, has become a 

 cell-problem, and its solution lies in a correct understanding 

 of the cell-phenomena involved. 



In the eighteenth century and early part of the nineteenth, 

 ideas concerning the sexual products and their relation to the 

 adult organism were vague and fanciful. The ablest anat- 

 omists and physiologists held that eggs agree in their structure 

 in every particular with the adult organism, and therefore 

 that they possess from the beginning the same organs arranged 

 in precisely the same manner and bearing the same relation 

 to each other, with the only difference that they are of ex- 



