SOME PROBLEMS OF COELENTERATE ONTOGENY 527 
the primordial germ-cells, are short or very long, whether they consist 
of three or six or sixteen cells, or of hundreds and thousands of cells. 
That all the cells of the germ-tracks do not take on the character of 
germ-cells must, in accordance with our conception of the maturing of 
determinants, be referred to the internal conditions of the cells and of 
the germ-plasm, perhaps in part also to an associated quantum of so- 
matic idioplasm which is only overpowered in the course of the cell 
divisions. This splitting up of the substance of the ovum into a somatic 
half, which directs the development of the individual, and a propaga- 
tive half, which reaches the germ-cells and there remains inactive, and 
later gives rise to the succeeding generation, constitutes the theory of 
the continuity of the germ-plasm (p. 411). 
Theoretically, the hypothesis is interesting and developed 
with much plausible argument. Yet its demonstration is far 
from evident, indeed quite beyond demonstration, as has been 
frequently pointed out by many of his critics. However, Weis- 
mann insists that there are evidential facts: 
The hypothesis does not depend for support merely on a recognition of 
its theoretical necessity, on the contrary there is a whole series of facts 
which may be adduced as strongly in its favor. Thus, even the familiar 
fact that excision of the reproductive organs in all animals produces 
sterility proves that no other cells of the body are able to give rise to 
germ-cells; germ-plasm cannot be produced de novo. 
It is passing strange that he should ignore the body of facts 
concerned in regeneration, and among them the reproductive 
organs. And it is still more strange that in support of this he 
should cite in detail the Hydrozoa as illustrating and supporting 
the hypothesis, ignoring the well-known facts that among these 
are abounding evidences which afford insuperable objections to 
just these assumptions. The present author has, in many cases, 
shown that gonads may be as readily regenerated by hydroids 
and medusae as any other organs; and that not for once or twice, 
but repeatedly in the same specimen, and that de novo and in 
situ; not the slightest evidence being distinguishable that any 
migration through pre-existing ‘germ-tracks’ occurred. The 
assumption that in these animals the gonads have ‘‘been shifted 
backwards in the course of phylogenetic evolution, that is, have 
been moved nearer to the starting point of development’’ seems 
so at variance with known facts as to be difficult to appreciate 
