INTRODUCTION 7 



body. And thus the wonderful truth became manifest that a single 

 cell may contain within its microscopic compass the sum-total of 

 the heritage of the species. This conclusion first reached in the 

 case of the female sex was soon afterwards extended to the male 

 as well. y Since the time of Leeuwenhoek (1677) it had been known 

 that the sperm or fertilizing fluid contained innumerable minute 

 bodies endowed in nearly all cases with the power of active move- 

 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 ? 

 Schwann and Schleiden held that cells might arise in two different 

 ways; viz. either by the division or fission of a pre-existing mother- 

 cell, or by " f ree cell-formation," new cells arising in the latter case 

 not from pre-existing cells, but by crystallizing, as it were, out of 

 a formative or nutritive substance, termed the " cytoblastema." It 

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



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

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

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



