196 THE CONTINUITY OF THE GERM-PLASM AS THE 



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 pre- 

 sent 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 de- 

 velopment 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 pro- 

 duced from it. Many writers appear to consider it a matter of 

 course that any embryonic cell can reproduce 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 segmentation 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 developed 

 animal body ? It is true that we speak of certain cells as being 

 ' of embryonic character,' and only recently Kolliker l has given a 

 list of such cells, among which he includes osteoblasts, cartilage 

 cells, lymph corpuscles, and connective tissue corpuscles : but even 

 if these cells really deserve such a designation, no explanation of the 

 formation of germ-cells is afforded, for the idioplasm of the latter 

 must be widely different from that of the former. 



It is an error to suppose that we gain any further insight into 

 the formation of germ -cells by referring to these cells of so-called 

 ' embryonic character,' which are contained in the body of the 

 mature organism. It is of course well known that many cells are 

 characterized by very sharply defined histological differentiation, 

 while others are but slightly differentiated ; but it is as difficult to 

 imagine that germ-cells can arise from the latter as from the 

 former. Both classes of cells contain idioplasm with a structure 



1 Kolliker, ' Die Bedeutung der Zellkerne,' etc. ; Zeitschr. f. wiss. Zool. Bd. XLII. 



