The germ-cells. 681 



credible. Why, for instance, should the germ-cells of the males of 

 a species arise from different cells and even from a different organ 

 and in a different locality from the female cells of the same species? 



XVI. Grerm-Cells and Emlbryo. 



According to current views the embryo forms the germ-cells, 

 which are the provision made for future generations. This belief is 

 the opposite of the truth. The germ-cells (that is to say, one of 

 them) give rise to the embryo. The rest of the germ-cells make 

 use of the latter as of a harbour of refuge for one portion of their 

 life-cycle. 



The prevailing, but erroneous, conception of the origin of the 

 germ-cells is, indeed, not without its counterpart in embryology. The 

 opinions maintained as to the origin of leucocytes and the nature of 

 the thymus furnish an exactly analogous case. For twenty years past 

 the majority of embryologists have believed and taught the "meso- 

 blastic" origin of leucocytes and their invasion of the epithelial 

 thymus. It is, perhaps, beside the point, that this dogma has from 

 start to finish been based on the slenderest evidence. It may suf- 

 fice, that it is utterly destitute of foundation. Leucocytes are not 

 mesoblastic but hypoblastic, they do not originate in mesoblast, and 

 thence invade the thymus, but they are the products of epithelial 

 cells, derived from hypoblast, and they invade the "mesoblast" from 

 the thymus. 



The two errors referred to above, relating to the germ-cells and 

 to the first leucocytes, illustrate the importance in research of a due 

 and careful consideration of the possibilities. For it may happen, 

 that the conclusion arrived at may be the very opposite of the 

 truth. 



Moreover, what is true of small things also holds for large ones. 

 Our conception of animal development as a whole and the inter- 

 pretations we place on its phenomena may exist merely in our imagi- 

 nations. An illustration in point would be the so-called phylogeuy of 

 the germ-cells. In the preceding pages any reference to this to many 

 embryologists most interesting subject has been carefully avoided, and 

 for two reasons. In the first place, any attempt to use the facts 

 recorded in the preceding pages in support of an hypothesis of the 

 "ancestral history" — save the mark — of the "germ-organs" would 

 be beset with insuperable difficulties, and, in the second, the con- 



