204 CONTINUITY OF THE GERM-PLASM AS THE [IV. 



through at any rate the main phases of their ontogeny. A 

 budden 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 transformation 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 know- 

 ledge 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 

 ultimate maturation of the sexual cells, the substance of the 

 nuclei returns to a condition similar to that which existed at 

 the beginning of ontogenetic development ; still such a state- 

 ment is no proof, but only an assumption made to support 

 a theor}'. 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 of the 

 idioplasm of this cell into germ-plasm. But this latter trans- 

 formation is just the point which most needs proof upon any 

 theory except the one which assumes that the primitive germ- 

 cell still contains unchanged germ-plasm. Every attempt to 

 render probable such a re-transformation of somatic nucleo- 

 plasm into germ-plasm breaks down before the facts known of 

 the Hydroids, in which only certain cells in the body, out of the 

 numerous so-called embryonic cells, are capable of becoming 

 primitive germ-cells, while the rest do not possess this power. 



I must therefore consider as erroneous the hypothesis which 

 assumes that the somatic nucleoplasm may be transformed into 

 germ-plasm. Such a view may be called 'the hypothesis of the 

 cyclical development of the germ-plasm.' 



Nageli has tried to support such an hypothesis on phyletic 

 grounds. He believes that phyletic development follows from 

 an extremely slow but steady change in the idioplasm, in the 

 direction of greater complexity, and that such changes only 



