The Animal Organism and its Germ-Layers 65 



as a particularly clear case of the genuine transformation of 

 endoderm cells into sex-cells. To be still more explicit, in 

 his figure (115 plate V,) is shown a sex-cell so slightly differ- 

 ent from the neighboring endoderm cells, and so related to 

 the surrounding cells, that, no one would hesitate to conclude 

 that it had very recently arisen by division of one of the 

 endoderm cells unless influenced by considerations other than 

 the evidence actually before his eyes. Exactly the same 

 conditions, he says, are observable in Clava, Sertularia, and 

 Scrtularella; and he then remarks: "From these observations 

 it becomes unfair to assume that in other cases where germ 

 cells arc found in the endoderm that they have wandered 

 from the ectoderm. Proof of this must be absolute." 8 The 

 full force of this remark is seen only when it is taken in 

 connection with Weismann's own statements about the sex- 

 cells of Corydendrmm para^siticum. He saw here no less 

 )sitively than did Goette, very young stages of the cells in 

 ie endoderm; but since, he says, he could not find the ab- 

 )lutely first stages "the possibility of the ectodermal ori- 

 gin of the sex-cells is not excluded." ! In other words, the 

 ?nce of absolute proof of the endodermal origin of the 

 Us is used to support the n /triori conclusion that they 

 irise in the ectoderm! So far as the observational evidence 

 this specific case goes, it strongly supports, as \Ycismann 

 limself grants, the conclusion that the reproductive cells 

 arise' in the endoderm, hut since the evidence falls a little 

 short of finality if may be cast aside wholly in favor of 

 purely theoretical grounds for supposing the origin to be 

 Isrwhere! Against this method of reasoning Goette 

 strongly protests, and every biologist who genuinely be- 

 Heves that speculative proof must yield to observational 

 proof when the two come into conflict, will say amen. And 

 when once one sees the extent to which Weismann's whole 

 system rests upon this method, and sees at the same time 

 how widely influential the system is, he will recognize that 



