94 
V. H. Blackman. 
sation has led to the revival of the old theory of Sachs’ of “ organ¬ 
forming substances.” This theory was put forward by Sachs in 
his article “ Stoff und Form der Pflanzenorgane,” as long ago as 
1880. Sachs here contends that the morphological nature of an 
organ is dependent upon the material substance from which it is 
built up. On such a view there must occur in a plant as many 
specific formative materials as there are different organs: there 
would be specific materials for foliage leaves, scale leaves, bracts, 
floral leaves, root, stem, stamens, carpels, etc. Sachs’ view 
obtained little support at the time, but of late years the theory of 
formative substances has been again brought forward by Conklin 
and Carl Rabl. They contend that the egg-protoplasm contains 
definite organ-forming substances which are necessary to develop¬ 
ment, so that the cytoplasm must be considered as a partner with 
the nucleus in transmitting hereditary properties. As Hertwig 
points out, the theory of these formative substances is such a vague 
one and so difficult to fit in with present-day biological conceptions 
that it can hardly be placed in serious opposition to the hypothesis 
of the nuclear idioplasm. It is not clear whether the substances 
are dead or living ; they are certainly not present in all egg-cells, 
and they do not seem to enable us to explain the processes of 
regeneration. Even when such formative substances are present, 
as apparently in the eggs of Ctenophora, they can be considered as 
having arisen during the maturation of the egg as a result of the 
activities of the female nucleus. 
Leaving the more general objections we may now review the 
chief objections to the theory on experimental and observational 
grounds. These fall mainly into two groups, those resulting from 
experiments on eggs and their fertilization, and those relating to 
the occurrence of simple amitotic divisions in plants and animals. 
It was at first hoped that experiments on merogony (in which 
an enucleate portion of egg-protoplasm is fertilised by the 
spermatozoon of another species or genus) would provide a crucial 
test for settling the question. The earlier classical experiments of 
Boveri, in which fragments of the eggs of Splicer echinus were fer¬ 
tilised with the spermatozoa of Echinus, turned out later to be 
inconclusive. For though small larvae were produced with purely 
paternal characters, it was shown that such forms might arise from 
normal hybrids of the two forms produced by fertilisation of a 
complete egg. Boveri’s conclusion that the absence of the egg- 
nucleus was the cause of the absence of maternal characters could 
