26 A. A. W. HUBRECHT. 
But more extensive investigations ad hoe will have to be undertaken before 
the isolated cases of the Reptilia above noticed will have obtained sufficient 
lateral support to serve as a starting-point on which a theory on the modifica- 
tion of the trophoblast in the Sauropsida—simultaneously with the formation 
of an eggshell, etc.—may be based.! 
Of the part played by the Sauropsidan trophoblast in the formation of the 
amnion we will have to speak in another chapter. Suffice it to add that no 
data are as yet available to determine the exact moment at which the plasmodi- 
trophoblast becomes distinguishable in the above-mentioned genera. Neither 
Mitsukuri nor Schauinsland give any indications. Furthermore, it would be 
important to know whether ontogeny gives any clue which would permit a guess 
as to the question whether the trophoblast has, in the viviparous ancestors of 
the Sauropsida, been as early differentiated from the remaining cleavage cells 
as is the case in mammals,? or whether the differentiation has only set in later 
as we find in the case of those Amphibia and fishes in which traces of an 
outer larval layer are also present, and which we will more fully discuss in 
the last paragraph of the next chapter. 
©. IcHTHYOPSIDS. 
In the paragraphs A and B of this chapter.we have 
attempted to show that beside the ectoderm and entoderm, 
which by delamination establish the gastrula stage of mammals 
and Sauropsids, there exists yet another very early cell-layer 
1 Recently Eternod has published an article, “La Gastrule dans la série 
animale,” in the * Bull. Soc. Vaud. Se. Nat.,’ 1906. 5e sér., vol. 42, in which, 
in text-fig, 16 and in fig. 26 on pl. 18, he attempts to homologise parts that 
are in no way homologous, if we look upon the early developmental processes 
of Mammals and Sauropsids in the light above advocated. Kterncd’s views 
have already been successfully protested against by Schlater (‘ Anat. Anz.,’ 
Bd. 31, p. 31). The latter author himself misses the mark, however, when he 
says that ‘die epiblastische Schicht der Sauropsiden-keimblase der iiber die 
Grenzen der Keimscheibe hinausgewaclisene embryonale Epiblast ist.” The 
secondary degenerative stages of the trophoblast are here wholly misunder- 
stood. 
2 The researches, above alluded to (pp. 12 and 20), of Wilson and Hill 
seem to imply that in Ornithodelphia we have yet an important intermediate 
stage, in which it is indeed possible, notwithstanding the yolk accumulation, 
to distinguish the trophoblast from the mother-cells of the embryonic knob. 
Semon’s (’94) figs. 33 and 34 allow of a similar interpretation. 
