412 
SUMMARY OF CURRENT RESEARCHES RELATING TO 
the general idea is that each period is the condition of its successor. 
Each blastomere has the power of responding to certain developmental 
stimuli, this is its “ prospective potency ” ; as organs arise this potency 
diminishes in their elements, the more so the later they are in being 
formed. 
The response of the nucleus to stimulus is conceived of as a fermen- 
tative effect on the cell-plasma, so that the reaction-power of the latter 
is altered. Again, nucleus and cytoplasm are, as it were, two poles, 
alternately active and receptive ; the plasma communicates an induction- 
stimulus to the nucleus, and receives in turn the fermentative response. 
But it is beyond us to condense this contribution to the philosophy of 
biology. From a natural mixture of experimental data and logical 
assumptions the author gradually works up to the mystery of the 
organism — its L threefold rhythm, its causal, structural, and functional 
harmony. 
Hertwig’s Theory of Development.* — Prof. J. Nusbaum criticises, 
but with a pleasing appreciation, Prof. Hertwig’s theory, of development. 
As is well known, Hertwig is “der hervorragende Yerteidiger des 
Neoepigenetismus ” the emphatic antagonist of Weismann, “der Schopfer 
des Neoevolutionismus.” 
Hertwig maintains that the ovum developes as it does, not merely 
because it is what it is, but because it grows in a certain environment, 
the subject of a metabolism which implies external relations, and that 
the result is determined from stage to stage by the mutual dynamic 
relations of the segmentation-cells and subsequent parts. 
He maintains that Anlage - product is by no means identical with the 
Anlage ; that the ovum may have particles which serve as Anlagen for 
horn, hair, bone, &c., but that it has not the Anlagen of a horn, a hair, 
a bone ; that it is in itself an organism. The endoderm cells are not 
invaginated because they possess certain groups of determinants ; rather 
they become endoderm because they are invaginated. The gastrulation 
is determined by the mutual influences of the germinal cells. 
Nusbaum regards this position as unfounded. With Weismann, he 
regards each step in development as the necessary result of the specific 
organisation of the germ-plasma. Gastrulation is a phylogenetic neces- 
sity, determined b.y the particular structure of the Anlage substance. 
He combats Hertwig’s position by citing cases where the endoderm, 
for instance, is plainly differentiated before the invagination. Such 
cases appear to Nusbaum to require Weismann’s interpretation — that in 
the early cleavages there is unequal partition of the hereditary qualities. 
For plants and lower animals, however, he believes in equal partition, 
which is also true in the first few cleavages of most higher animals. 
To Nusbaum the external conditions are simply stimuli, not operative 
factors ; they allow the nature of the Anlage to assert itself. Similarly, 
the mutual influences of the cells are merely stimuli which call the 
Anlagen in the germ-plasm of the associated cells to activity and 
development. One can hardly help feeling that the truth is likely to 
be in the combination of the doctrines of the two schools, with an 
elimination of the exaggerations on both sides. 
* Biol. Centralbl., xv. (1895) pp. 286-94. 
