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. ^ As 

 long ago as i_286i_ however, it was shown by Spalla nzani that t he 

 fertilizin^_j3ower must lie in the spermatozoa, noMn jthe..li_guid 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_Xi84JJ that the spermatozoa arise 

 directly frQm _ 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 Kolhker believed, but also cytoplasm. It 

 was thus shown to be, like the Q.g%, 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 O scar Hertwi g 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 ccll-probloiis. 



Meanwhile, during the years immediately following the announce- 

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

 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 vo n 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). 



