200 THE CONTINUITY OF THE GERM-PLASM AS THE 



Monotremata, mammal-like reptiles, and the lower vertebrate forms, 

 into worms and finally into Monera. But how can a course of 

 development, which seems to be impossible in phylogeny, occur as 

 the regular method of ontogeny ? And quite apart from the 

 question of possibility, we have to ask for proofs of the actual 

 occurrence of cyclical development. Such a proof would be af- 

 forded if it could be shown that the nucleoplasm of those somatic 

 cells which (e.g. in Hydroids) are transformed into germ-cells 

 passes backwards through many stages of development into the 

 nucleoplasm of the germ-cell. It is true that we can only recognise 

 differences in the structure of the idioplasm by its effects upon the 

 cell-body, but no effects are produced which indicate that such 

 backward development takes place. Since the course of onward de- 

 velopment is compelled to pass through the numerous stages which 

 are implied in segmentation and the subsequent building-up of the 

 embryo, etc., it is quite impossible to assume that backward develop- 

 ment would take place suddenly. It would be at least necessary to 

 suppose that the cells of embryonic character, which are said to be 

 transformed into primitive germ -cells, must pass back through at any 

 rate the main phases of their ontogeny. A sudden transformation 

 of the nucleoplasm of a somatic cell into that of a germ-cell would 

 be almost as incredible as the transformation of a mammal into an 

 amoeba ; and yet we are compelled to admit that the transforma- 

 tion must be sudden, for no trace of such retrogressive stages of 

 development can be seen. If the appearance of the whole cell gives 

 us any knowledge as to the structure of its nuclear idioplasm, we 

 may be sure that the development of a primitive germ-cell proceeds 

 without a break, from the moment of its first recognizable formation, 

 to the ultimate production of distinct male or female sexual cells. 



I am well aware that Strasburger has stated that, in the ulti- 

 mate maturation of the sexual cells, the substance of the nuclei 

 returns to a condition similar to that which existed at the begin- 

 ning of ontogenetic development ; still such a statement is no 

 proof, but only an assumption made to support a theory. I am 

 also aware that Nussbaum and others believe that, in the formation 

 of spermatozoa in higher animals, a backward development sets in 

 at a certain stage ; but even if this interpretation be correct, such 

 backward development would only lead as far as the primitive germ- 

 cell, and would afford no explanation of the further transformation 



