364 THE AMERICAN NATURALIST. (VoL. XXXIII. 
“With respect to the first objection, it might be argued, with a 
good deal of plausibility, that the term ‘hypoblast’ is used to denote 
that surface which is known by its later development to form the 
alimentary canal, that in fact, as Heymons has pointed out, the 
theory of the germinal layers is not sufficiently well established to 
give it any phylogenetic value.” Are we to understand from this 
that since the canalis centralis does not develop into an alimentary 
canal, it is probably hypoblastic? 
“ 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 
FIG. 3. Fic. 4. 
Fics. 3, 4. — Cross-sections of a vertebrate in the head r on, ill brate cond 
tion may be attained by turning the arthropod over on to its dorsal icine: thus oi ds ae its 
principal organs into the same relative position as in vertebrates. No pce of old 
be or formation of new ones is snag septa 3 to this view. ane pithelium of the 
centralis and a thus appear not as parts of an old ali canal, but as the 
infolded ectoderm that from the first overlies pe brain caf spinal cord, oud from which they 
are phylogenetically as well as ontogenetically derived. 
central canal. ... 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 (ż.e., the ventral epiblast of the 
arthropod) and the neural canal (ż.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 plates of the mesoblast, we are 
watching the embryonic representation of the original Limulus-like 
animal; when the lateral plates of the mesoblast have grown round, 
