TRANSACTIONS OF SECTION I. 949 
this question, seeing that Heymons has shown that the whole alimentary tract in 
such arthropods as the earwig, cockroach, and mole cricket, is, like the nerve tube 
of vertebrates, formed from epiblast. 
The second objection appears to me more apparent than real. The nerve layer, 
in the vertebrate, as soon as it can be distinguished, is always found to lie ventrally 
to the layer of epiblast which forms the central canal. In the middle line of the 
body, owing to the absence of the mesoblast layer, the cells which form the noto- 
chord and those which form the central nervous system form a mass of cells which 
cannot be separated in the earlier stages. The nerve layer in the arthropod lies 
between the ventral epiblast and the gut; the nerve layer in the vertebrate lies 
between the so-called hypoblast (7.e. the ventral epiblast of the arthropod) and 
the neural canal (7.e. the old gut of the arthropod). The new ventral surface of 
_ the vertebrate in the head region is not formed until the head fold is completed. 
- Before this time, when we watch the vertebrate embryo lying on the yolk, with its 
nervous system, central canal, and lateral pee of mesoblast, we are watching the 
embryonic representation of the original Limulus-like animal; then, when the 
lateral plates of mesoblast have grown round, and met in the middle line to assist 
in forming the new ventral surface, and the head fold is completed, we are watching 
the embryonic representation of the transformation of the Limulus-like animal into 
the scorpion-like ancestor of the vertebrates. 
In the Arthropoda, the simple epithelial tube which forms the stomach and 
intestine is not a glandular organ, and we find that the digestive part of the ali- 
mentary tract is found in the large organ, the so-called liver. This organ, together 
with the generative glands, forms an enormous mass of glandular substance, which, 
in Limulus, is tightly packed round the whole of the central nervous system and 
alimentary canal, along the whole length of the animal (represented in fig. 4 by 
the dark dotted substance). The remains of this glandular mass are seen in 
Ammoceetes in the peculiar so-called packing tissue around the brain and spinal 
cord (represented in fig. 6 by the dark dotted substance), It satisfies the three 
tests to the following extent :— 
1. The phylogenetic test.—As we descend the vertebrate phylum, we find that 
the brain fills up the brain-case to a less and less extent, until finally in Ammoccetes 
a considerable space is left between brain and brain-case, filled up with a peculiar 
glandular-looking material, interspersed with pigment, which is not fat tissue, and 
is most marked in the lowest vertebrates. The natural interpretation of this 
phylogenetic history is that the cranial cavity is too large for the brain in the 
lowest vertebrates, and is filled up with a peculiar glandular substance because 
that glandular substance pre-existed as a functional organ or organs, and not 
because it was necessary to surround the brain with packing material in order to 
keep it steady, owing to the unfortunate mistake having been made of forming a 
_ brain much too small for its case. : 
2. The anatomical test shows that this glandular and pigmented material is in 
the same position with respect to the central nervous system of Ammoccetes as the 
generative and liver material with respect to the central nervous system and 
alimentary canal of Limulus. 
3. The ontogenetic test remains to be worked out. Ido not know the orgin of 
this tissue in Ammoccetes ; the evidence has not yet been given by Kuppfer.t. He 
has, however, shown that the neural ridge gives origin to a mass of mesoblastic 
cells, the further fate of which is not worked out. The whole story is very sugges- 
tive from the point of view of my theory, but incomprehensible on the view that 
the neural ridge is altogether nervous. 
Finally, we ought to find in the invertebrate group in question indica- 
tions of the commencement of the enclosure of the alimentary canal by the 
central nervous system; such is, in fact, the case. In the scorpion group a 
marked process of cephalisation has gone on, so that the separate ganglia, 
both of the prosomatic and mesosomatic region, have fused together, and fused 
‘ Kuppfer, Studien 2. vergleich. Entwicklungsgesch. d. Kopfes der Kranioten 
2. Heft, Miinchen u. Leipzig, 1894. 
