1897 - 98 .] Dr Mas ter man on Ar chimeric Segmentation. 277 
be intermediate in character. The reproductive cells will, there- 
fore, he no longer found in the protoccele, hut will be confined to 
the metacceles ; whilst, on the other hand, the protoccele will lose 
the reproductive function and will he entirely devoted to muscular 
differentiation. In other words, the protocoele will become more 
‘ animal ’ or katabolic in form and function, the metacoeles more 
anabolic. One curious anatomical peculiarity follows from this — 
the circulatory vascular system not being as yet fully differentiated 
the main excretory organ will be in connection with the protocoele, 
where the metabolism is most active, and not as in the highest 
animals, in the metacceles, which in them form the c animal 5 organs. 
I have attempted to show elsewhere* that the function of excre- 
tion primitively arises in the ectoderm, and in this hypothetical 
ancestor, the ectoderm of the stomodseum probably came into close 
connection by an invagination with the protocoele and the vascular 
sinus in its neighbourhood. This primitive excretory organ may 
be termed the subneural gland. We have now two important sets 
of organs the evolution of which are yet to be followed, namely, 
the vascular and the nervous systems. They may well be taken 
together, for they are intimately connected. In the paper already 
referred to, an attempt has been made to show that the vascular 
system arises phyletically as a system of spaces between the limiting 
epithelia, ectoderm and endoderm, and the mesoderm, and that the 
vascular fluid is primarily excretory in function. The nervous 
areas being, by their very nature, areas of active metabolism, their 
course is largely followed by the vascular vessels immediately below 
them. 
In Stage I. the nervous system must have been upon a radial 
principle, and although, as in the present day pelagic Coelentemta , 
consisting of a diffuse nervous plexus in continuity with the ecto- 
dermal epithelium, was probably concentrated in certain areas in 
relation to the ciliary ingestive and locomotory organs, and had a 
tatramerous arrangement, four inter-radial nerves meeting a rhm 
round the edge. Where the four radial nerves meet at the aboral 
pole it is reasonable to suppose, from the analogy of Ctenophora , 
that a central ganglion would be developed. Immediately below 
this ganglion would be the principal blood-sinus (subneural sinus), 
* Zool. Anze'g., 501 - 503 . 
