IV.] FOUNDATION OF A THEORY OF HEREDITY. 1 99 



and most animals are not, as a matter of fact, separated from 

 the somatic cells at the beginning of ontogeny; the latter, 

 because it is contradicted by the fact that the sexual cells of 

 vertebrates do not arise from blood corpuscles, but from the 

 germinal epithelium. But even if this fact had not been ascer- 

 tained we should be compelled to reject Valaoritis' hypothesis 

 on theoretical grounds, for it is an error to assume that white 

 blood corpuscles are undifferentiated, and that their nucleo- 

 plasm is similar to the germ-plasm. There is no nucleoplasm 

 like that of the germ-cell in any of the somatic cells, and no one 

 of these latter can be said to be undifferentiated. All somatic 

 cells possess a certain degree of differentiation, which may be 

 rigidly limited to one single direction, or may take place in one 

 of many directions. All these cells are widely different from 

 the egg-cell from which they originated : they are all separated 

 from it by many generations of cells, and this fact implies that 

 their idioplasms possess a widely different structure from the 

 idioplasm, or germ-plasm, of the egg-cell. Even the nuclei of 

 the two first segmentation spheres cannot possess the same 

 idioplasm as that of the first segmentation nucleus, and it is, of 

 course, far less possible for such an idioplasm to be present 

 in the nucleus of any of the later cells of the embryo. The 

 structure of the idioplasm must necessarily become more and 

 more different from that of the first segmentation nucleus, as 

 the development of the embryo proceeds. The idioplasm of 

 the first segmentation nucleus, and of this nucleus alone, is 

 germ-plasm, and possesses a structure such that an entire 

 organism can be produced from it. Many writers appear to 

 consider it a matter of course that any embryonic cell can re- 

 produce the entire organism, if placed under suitable conditions. 

 But, when we carefully look into the subject, we see that such 

 powers are not even possessed by those cells of the embryo 

 which are nearest to the egg-cell — viz. the first two segmenta- 

 tion spheres. We have only to remember the numerous cases 

 in which one of them forms the ectoderm of the animal while 

 the other produces the endoderm, in order to admit the validity 

 of this objection. 



But if the first segmentation spheres are not able to develope 

 into a complete organism, how can this be the case with one of 

 the later embryonic cells, or one of the cells of the fully de- 



