436 WILSON. mm [Vere 
massive sphere of yolk in such a manner as to meet along the 
median ventral line. TZhzs whole process of ctrcumcrescence and 
concrescence has arisen secondarily, in adaptation to fetal con- 
ditions that do not exist in the larval form. The blastopore, if 
we include the space traversed in the closure of the germ-bands, 
has been stretched out of all proportion to its original dimen- 
sions, so that it no longer represents the primitive Gastrula- 
mouth, but merely a secondary prolongation of it backwards 
along the whole ventral line of the body. In the embryonic 
Trochosphere we find the blastopore already closed before the 
trunk-bud begins to develop; hence the line of closure (‘ Gas- 
trula-raphe”’) is limited to the ventral line of the Trochosphere. 
As the metameric body-region is not yet developed, it is evident 
that the posterior limit of the primitive blastopore falls within 
the non-metameric region, from which the head-segment of the 
adult animal is formed.” 
Concrescence, as thus conceived, is, therefore (to use Professor 
Whitman's own expression), a process of restoration, by which 
the two halves of the embryo, which have been mechanically 
separated by the backward extension of the blastopore along the 
median ventral line, are brought together again. But a broader 
examination of the question demonstrates the inadequacy of this 
explanation, clear as it at first sight appears to be. It is per- 
fectly obvious that the history of the mesoblastic and neural 
elements in Lumbricus is essentially the same as in Clepsine and 
other epibolic foetal types. In both the mesoblastic and neural 
bands are at first widely separated throughout the middle region 
of the trunk, and subsequently undergo a process of union along 
the median ventral line. Lambricus differs only in the accelera- 
tion of the ectoblastic part of the germ-bands, which outstrips 
the other elements and thus closes in the blastopore while the 
neural and mesoblastic elements still lie quite at the sides of 
the embryo. Yet the embryo contains very little food-yolk ; the 
gastrulation is of the embolic type, and hence the separation 
and subsequent concrescence of the germ-bands cannot be 
explained under Whitman’s view, unless we suppose the gas- 
trulation to be a secondary derivative of an epibolic form. 
In view of such a possibility, however, let us waive the case of 
Lumbricus and consider the typical larval types. No one can 
compare the history of the mesoblastic bands in C/epszne, in 
