370 PROGRESS OF SCIENCE IN THE CENTURY. 



gins its life as a cell ; in short, that the egg is a cell. 

 Somewhat later (1841) Kolliker traced the spermat- 

 ozoa to their origin in the essential male organs or 

 testes, and it was soon recognised that the spermato- 

 zoon also is a cell. We now know that both ovum 

 and spermatozoon may show a complexity of minute 

 organisation which was not suspected in the first half 

 of the century, but this after all is a matter of detail. 

 The fundamentally important fact, which differ- 

 entiates modern embryological conceptions from 

 those of the first half of the nineteenth century is 

 the idea of genetic continuity. This may be espe- 

 cially traced to the work of Yirchow (1858), though 

 several others were approaching it about the same 

 time. 



" To the modern student the germ is, in Huxley's 

 words, simply a detached living portion of the sub- 

 stance of a pre-existing living body carrying with it a 

 definite structural organisation characteristic of the 

 species." * In other words, an egg or a sperm liberated 

 from or set apart in any organism is connected by a 

 lineage of cell-divisions with the fertilised ovum which 

 gave rise to that organism, and so on backwards. It 

 was an epoch-making step when embryologists arrived at 

 "the conception so vividly set forth by Virchow of an 

 uninterrupted series of cell-divisions extending back- 

 ward from existing plants and animals to that remote 

 and unknown period when vital organisation assumed 

 its present form. Life is a continuous stream. The 

 death of the individual involves no breach of continuity 

 in the series of cell-divisions by which the life of the 

 race flows onwards. The individual body dies, it is 

 true, but the germ-cells live on, carrying with them, as 



* E. B. Wilson, The Cell in Development ana in Inherit- 

 ance, 3nded., 1900, p. 7. 



