No. 3.} ZTHE EMBRYOLOGY OF THE EARTHWORM. 435 
dynamous, not with the double cavity of a single somite, but 
with one of the patr of cavities, by the fusion of which each 
celomic cavity of the trunk arises ; in other words, the coelomic 
cavities, with their mesoblastic walls, can be most simply and 
accurately conceived as forming an elongated ring, the two 
halves of which lie at the sides of the alimentary canal, and are 
connected in front by the head-cavity, the walls of which become 
differentiated into the parts of head! Can this be regarded as 
a characteristic of the ancestral body? If so, the retardation of 
the jtrunk-region in the larval Trochosphere is obviously a 
secondary feature of development, and the head cannot be 
regarded as older than the trunk. Under this view, moreover, 
it would follow that the concrescence of the mesoblastic and 
neural bands that is so striking a feature of annelidan develop- 
ment must be regarded as an ancestral feature, and not as the 
result of secondary changes caused by special embryonic con- 
ditions. This question I shall now briefly discuss. 
XII. ConcrRESCENCE AND THE BLASTOPORE. 
We are indebted to Whitman for a most interesting analysis 
of the phenomena of concrescent growth in the annelid embryo, 
the process being conceived as a secondary feature of develop- 
ment, brought about by accumulation of deutoplasm in the 
ovum. His general conclusions (in respect to C/epszne) are 
stated in the following passage (No. 52, p. 175; the italics are 
mine) :— 
“Among the more important differences remaining to be 
noticed are those which have been brought about under the 
influence of the food-yolk. The process of gastrulation, the 
form of the blastopore and its relations to the mouth, have been 
very profoundly modified in this way. The trunk-bud of the 
foetal Trochosphere has been carried far from its original post- 
oral position; and, as the result of this displacement, we see 
the halves (germ-bands) of the trunk (which develop side by side 
as a unit in the larva) formed separately and carried over the 
1] may point out the fact that the ccelom of annelids is generally agreed to be 
primitively divided by dorsal and ventral mesenteries into right and left cavities, 
though the dorsal mesentery often disappears; and that the discovery of the excretory 
organs of the head has still further lessened the supposed contrast between head and 
trunk. 
