CONTRIBUTIONS TO THE EMBRYOLOGY OF NEMERTEA. 421 
These formative mesoblast cells, undoubtedly performing 
amceboid movements in the blastocceel, into which they have 
been set free, must now be studied somewhat more in detail. 
When once they are met with inside the segmentation cavity 
it is impossible to decide with certainty from which of the two 
primary cell layers each of them has taken its origin. 
Nevertheless, we can hardly doubt, on theoretieal considera- 
tions, that certain differences of an essential hereditary nature 
must obtain between them. In tracing the origin of the 
nervous system we will further consider this problem. It may 
now suffice to call in mind the fact that of late years most 
important cytological researches have forced us to the con 
clusion that the nucleoplasma is of the highest importance in 
cell division, and that the life-history of certain cell groups, 
which, combined into distinct tissues, may be said to be 
determined by the nature of the nucleoplasm of the embryonic 
mother-cell from which they have all originated (cf. Weismann, 
‘Ueber die Continuitat des Keimplasmas,’ 1885). If this be 
true other hereditary determinants must be inherent in the 
epiblastic mesoblast cells than in the hypoblastic. A fact 
which must be considered in connection with this, and which 
probably has an important cytological significance (it was 
more fully described by me, l.c., p. 10), is that the chromatic 
nuclear substance of the primary epiblast diminishes, even 
when its surface increases, towards the time when it is going 
to be cast off. There is thus a decrease in the significance of 
the primary epiblast as a formative element for the moulding 
of the young larva inside it, which becomes more and more 
marked as the latter increases in size. Instead of holding 
that the nuclear substance which has disappeared from this 
arises independently of the four plates or “discs” of secondary epiblast, 
and that moreover the number of these discs contributing towards the forma- 
tion of the body wall, is not four but five, or even six (if we include the 
independent origin of the proboscidian epithelium), it need hardly be insisted 
that these comparisons, however ingenious (l.c., p. 279—281, 288), lose all 
foundation. Bergh’s suggestion that the esophagus of Lineus might arise 
by the coalescence of the cephalic and ventral plates (1. c., p. 281) is, moreover, 
negatived by my own results (see below). 
