THE FORMA'l'ION OK CERM-CEI.T.S 1 83 



CHAPTER VI 



THE FORMATION OF GERM-CELLS 

 I. The Continuity of the Germ-plasm 



If heredity depends on the presence of a substance, the 

 germ-plasm, which causes the production of the new individual 

 by directing the process of division in ontogeny, in the course of 

 which it becomes changed in a regular manner, the question 

 arises as to how unaltered germ-plasm can nevertheless reappear 

 in the orerm-cells of the new individual. The transmission of 

 characters from parent to child can only depend on the germ- 

 cell from which the offspring arises containing ids of germ-plasm 

 precisely similar to those of the germ-cell from which the parent 

 was developed. The germ-plasm, however, undergoes an enor- 

 mous number of changes during the development of the ovum 

 into the parent : how is it possible therefore that this substance 

 can reappear in the germ-cells of this parent? 



There are obviously two possible solutions of this problem. 

 The changes which the germ-plasm undergoes during the con- 

 struction of the body must either be of such a kind that they 

 can take place in the reverse order when the idioplasm of all, or 

 at least of a portion of, the somatic cells is re-transformed into the 

 germ-plasm from which it was, in fact, indirectly derived ; or, if 

 such a reversal is impossible, the germ-plasm of the germ-cells 

 must be handed on directly from parent to offspring. This 

 latter hypothesis was suggested by me some years ago under the 

 name of the contimiity of the germ-plasm.* A third solution of 

 the problem is impossible, for it is quite out of the question that 

 the germ-plasm can be entirely formed anew. 



The hypothesis of the continuity of the germ-plasm depends 

 on the assumption of a contrast between the somatic and the 

 reproductive cells, such as can be observed, in fact, in all multi- 

 cellular plants and animals, from the most highly differentiated 

 forms to the low-est heteroplastids amongst the colonial Algae. 



* ' Die Continuitat des Keimplasma's als Grundlage einer Theorie der 

 Vererbung; Jena, 1885 (English translation, 2nd ed., p. 163). 



