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RICHAED ASSHETON. 
and plasmodi-troplioblast. But where, even in Erinaceus or 
Vespertilio which he cites, can a plasmodiblast layer be found 
as a continuous thin layer of squamous cells ? The character 
of the layer as drawn by Schaninsland is that of an ordinary 
siiperficial layer such as may be found on the embryo them- 
selves of Sauropsida and Mammalia alike. 
Nevertheless pages 19-26 ai*e by no means the least inter- 
esting of Hubrecht’s stimulating work. 
(3) The comparison of the outer layer of the blastocyst of 
Prototheria and Metatheria (which is admittedly developed in 
a very diffei*ent manner) to the trophoblast of Eutheria seems 
to me, with all respect to his great authority, to be based on 
the slenderest of foundations. 
This leaves ns with the traces of larval envelope found in 
Dipnoi and Ganoids and Teleosteans. There is certainly in 
Dipnoi a separation into the layers of epiblast, though not so 
distinct a one as in the Anura, and its subsequent history is 
not like that of the Anura with reference to the central 
nervous system. 
Graham Kerr (1903) (PI. 4, fig. 18, ‘ Quart. Journ. Micr. Sci.,’ 
vol. 43), shows a neural groove into which the outer layer of 
epiblast passes, but the groove is very shallow, and although 
a chink is present for a short time (fig. 19) in which some of 
the outer layer cells might be imbedded, it is not at all pro- 
bable that the outer layer takes any part in the formation of 
the central nervous system. Graham Kerr says : ‘^The whole 
thickening of the keel is confined to the deep layer of the 
ectoderm — the outer layer passing unaffected over the floor 
of the groove.” 
But neither in Ceratodus nor Lepidosiren does the distinc- 
tion seem to be of the same nature as in Teleostei. 
In the Teleostean the condition is far more like that of the 
trophoblast of the Eutherian mammals than is any other of 
the supposed larval layers mentioned by Hubrecht. It clearly 
is not concerned structurally in the embryo fonnation, and 
being continuous with the periblast — indeed part of the peri- 
blast — it may be said to be trophic. Also it separates off 
