No. 388. ] CORRESPONDENCE. 363 
of “sub-arachnoidal glandular tissues,” that helps fill up the cranial 
cavity, and thus keeps the brain in place. The straight intestine is 
discovered under the guise of the central canal of the spinal cord 
and the “ cephalic stomach”’ as the ventricular cavities of the brain. 
These are only a few of the renovations to which the crabs must 
submit in their efforts to become vertebrates. There is not much 
left that can be used to advantage in the “ new crab,” except the 
skin and bones, and Gaskell makes a good deal of them, although 
he does not display as much ingenuity in doing so as in some of the 
instances quoted above, because the essential points of resemblance 
between the cartilages and the dermal structures of Limulus and those 
of vertebrates have already been pointed out bythe author. Gaskell 
hopes to explain some time how our eviscerated ancestors acquired 
a new heart, kidneys, and productive organs, as well as a new 
alimentary canal complete, from mouth to anus. 
Gaskell argues that he is justified in “ violating the embryological 
unities,” as he calls them, on the grounds that there is much scepticism 
abroad concerning the validity of the germ layer theory. But this 
weakness of the germ layer theory can hardly be construed as a 
license to transfer a crab’s entire alimentary canal and nervous 
system from the ventral to the dorsal surface without obtaining 
some authority from established facts, yet the assumption that this 
transfer has taken place forms the foundation of Gaskell’s theory. 
‘The conception is untenable, not because the supposed transfer of 
organs is a novel and surprising one, or because it is difficult to see 
how the animal could survive the operation, but because it assumes 
a condition of affairs that does not exist, because the peripheral 
nerves and the cross commissures present impassable barriers to the 
proposed changes, and because the suggestion is inconsistent with 
the most obvious facts of embryology. 
In 1896 (Nature), about eight years after the idea first occurred 
to him, Professor Gaskell is led to suspect that there may be some 
difficulties in homologizing the hzmal surface of a crab with the 
neural surface of a vertebrate, and gives the following explanation : 
The ontogenetic test appears to fail in two points: 
(1) “That the nerve tube of vertebrates is an epiblastic tube, 
whereas, if it represented the old invertebrate gut, it ought to be 
largely hypoblastic. 
(2) “The nerve tube of vertebrates is formed from the dorsal 
surface of the embryo, while the central nervous system of arthro- 
pods is formed from the ventral surface. 
