EAELY ONTOGENETIC PHENOMENA IN MAMMALS. 237 
cells of the brain and spinal cord, while the “ GrnndSchicht ” 
becomes the nervons tissue. It is, in fact, a separation into 
what His termed “ spongioblastic ” and “ neuroblastic ” tissue. 
The early stages of this process are described in a paper 
by myself with figures in vol. 37 of this Journal, pp. 166-169 
and PI. 18. 
To summarise the present section I would urge that 
Hiibrecht has not established the homology of the envelopes 
he mentions either among themselves or to the trophoblast 
of Eutherian mammals, because : 
(1) The epidermic layer of the Anura, upon which great 
reliance is laid for the support of the theory (p. 81), is neither 
morphologically nor physiologically similar to the trophoblast 
of Eutherian mammals, but is in fact the result of an early 
separation of spongioblastic from neuroblastic elements of the 
ectoderm. 
(2) The vestiges of an outer envelope which Hnbrecht 
retains as instances of larval envelopes among the Sauropsida 
differ from those he rejects as epitrichial only in the time of 
their development. The former appear before amnion forma- 
tion, the latter after. Quite similar vestiges may be found 
among the Mammalia which render the comparison uncon- 
vincing, because the real trophoblast in the Mammalia is 
obviously present elsewhere. 
It is curious that Dendy (’99) should make no mention of 
the occurrence of this layer in Sphenodon. But if we take 
Schauinsland’s figures as being correct and regard the 
presence of this layer as a well-established fact, we cannot 
compare it with the deckschicht of Anura because it takes no 
part in the formation of the embryo, nor with the Teleostean 
layer because it is quite Hee from the yolk-mass. 
A comparison with the Eutherian trophoblast is hardly less 
difficult. Hnbrecht attempts to do so by comparing the 
whole extra-embryonic epiblast of Sauropsida with the 
trophoblast of Eutherian mammals by supposing the double- 
layered condition of, for instance, Chameleo (Schauinsland, 
’03, figs. 182-219) to be a separation into cyto-trophoblast 
VOL. 54, PART 2. — NEW SERIES. 17 
