No. 2.] THE MESODERM AND MESENCHYM. 365 
the majority of forms, —pseudoepithelia of these primitive 
sexual cells arrange themselves in the form of paired and meta- 
meric sacks, which are then the gonads. Thus the comparison 
holds good, that in both Annelids and Nemerteans the sexual 
products are derived from the lining of the perivisceral body 
cavity. Inthe Metanemerteans the blood-vessels are completely 
closed, while in the lower Nemerteans they are (in the head 
region) in communication with the pseudocoelic spaces: in the 
former as in the Chaetopoda, in the latter as in the Hirudinea. 
Thus parts of the body cavity in the Nemerteans may be com- 
pared to a true coelom, other portions to a pseudocoel (archi- 
coel). The great difficulty of determining the homology of the 
Nemertean body cavity is simply due to the fact that it unites 
characteristics of a coelom and a_pseudocoel — formations 
which, according to the mesenchym theory of Hertwig, had 
formerly been supposed sharply distinguishable. So we find 
the difficulty is in reality owing to the fact of the impossibility 
of correctly homologizing such structures as body cavities ; for 
not only may two (supposedly) different types of cavities be 
present in the same species, but the two are frequently so 
intermingled in it as to render their recognition almost im- 
possible in our present state of knowledge. Thus we are not 
in position to state whether the Nemertean body cavity is of 
Turbellarian or of Annelidan character. 
The general conclusion which I would maintain, then, is that 
body cavities in different animal groups cannot be safely homol- 
ogized, either from the ontogenetic or from the comparative 
anatomical standpoint; though the latter method would seem 
to be, in this matter at least, more reliable than the preceding. 
The body cavity, whether as coelom or as pseudocoel, is not 
comparable to any single organ or set of organs, but must be 
considered as a structure of approximately equal economy to 
all the organs. And in its early formation and later differ- 
entiation probably most if not all of the organs take part, 
beginning with the blastomeres as the earliest. Accordingly, 
before seeking to homologize the body cavity, the morpho- 
logical value of all the organs themselves must be learned, as 
well as the morphological value of their mutual topography. 
