INTRODUCTION 9 



ment, and therefore regarded by the early observers as parasitic 

 animalcules or infusoria, a view which gave rise to the name sperma- 

 tozoa (sperm-animals) by which they are still generally known. 1 As 

 long ago as 1786, however, it was shown by Spallanzani that the 

 fertilizing power must lie in the spermatozoa, not in the liquid in 

 which they swim, because the spermatic fluid loses its power when 

 filtered. Two years after the appearance of Schwann's epoch-mak- 

 ing work Kolliker demonstrated (1841) that the spermatozoa arise 

 directly from cells in the testis, and hence cannot be regarded as 

 parasites, but are, like the ovum, derived from the parent-body. Not 

 until 1865, however, was the final proof attained by Schweigger- 

 Seidel and La Valette St. George that the spermatozoon contains 

 not only a nucleus, as Kolliker believed, but also cytoplasm. It 

 was thus shown to be, like the egg, a single cell, peculiarly modified 

 in structure, it is true, and of extraordinary minuteness, yet on the 

 whole morphologically equivalent to other cells. A final step was 

 taken ten years later (1875), when Oscar Hertwig established the 

 all-important fact that fertilization of the egg is accomplished by 

 its union with one spermatozoon, and one only. In sexual repro- 

 duction, therefore, each sex contributes a single cell of its own body 

 to the formation of the offspring, a fact which beautifully tallies 

 with the conclusion of Darwin and Galton that the sexes play, on 

 the whole, equal, though not identical parts in hereditary trans- 

 mission. The ultimate problems of sex, fertilization, inheritance, 

 and development were thus shown to be cell-problems. 



Meanwhile, during the years immediately following the announce- 

 ment of the cell-theory, the attention of investigators was especially 

 focussed upon the question : How do the cells of the body arise ? 

 The origin of cells by the division of preexisting cells was clearly 

 recognized by Hugo von Mohl in 1835, though the full significance 

 of this epoch-making discovery was so obscured by the errrors of 

 Schleiden and Schwann that its full 'significance was only perceived 

 long afterward. The founders of the cell-theory were unfortunately 

 led to the conclusion that cells might arise in two different ways, viz. 

 either by division or fission of a preexisting mother-cell, or by " free 

 cell-formation," new cells arising in the latter case not from pre- 

 existing ones, but by crystallizing, as it were, out of a formative or 

 nutritive substance, termed the " cytoblastema " ; and they even 

 believed the latter process to be the usual and typical one. It 

 was only after many years of painstaking research that " free cell- 

 formation " was absolutely proved to be a myth, though many of 



1 The discovery of the spermatozoa is generally accredited to Ludwig Hamm, a pupil 

 of Leeuwenhoek (1677), though Hartsoeker afterward claimed the merit of having seen 

 them as early as 1674 (Dr. Allen Thomson). 



