1891.] Embryology. 165 
(also the lateral wings of the mesoderm) is formed from the phylo- 
genetic endoderm, and may be compared to the formation of noto- 
chord’and mesoblast of Batrachia. 
We have sufficient evidence to believe that between the Batrachia 
and the Mammalia a ‘‘ phylogenetic link has once existed, in which the 
actual food-yolk formed a very considerable addition to the early 
blastocyst. The case of the Ornithodelphia is most important in this 
‘respect.. . . When the nutritive contents of the yolk-sac were no 
longer of primary importance, . . . a reduction in size of the blasto- 
cyst was not effectuated because another factor came into play, The 
vascular area which heredity called forth on the surface of the yolk-sac 
. . . must have rendered eminent service for the establishment of a 
different mode of nutrition, as soon 
as the embryo underwent a con- 
siderable part of its development 
inside the maternal generative 
.”’ Hence the large size of the 
blastocyst of the mammal has been 
retained not because it once contained 
yolk, but because it was an essential 
function to perform in the nutrition 
of the embryo. 
The accompanying diagram (Fig. 
3) represents (somewhat modified) 
the author’s figure to show the 
relationship to each other of the : 
mammalian germ-layers. The greater part of the central (fluid) 
cavity is surrounded by two layers,—the outer of ectoderm, the tropho- 
blast, and within the ontogentic endoderm. The upper part of the 
` figure shows the embryonic layers. The endoderm passes under the 
ectodermal embryonic shield (black). The posterior part of the lat- 
ter (with white streaks) shows the area of the primitive streak, and 
from this runs forward under the embryonic shield a prolongation 
(no.ch., black with white dots), forming the posterior part of the reset 
chord, and laterally, though not shown in the figure, the wings 
of mesoblast, In front of this is seen a thickened part of the onto- 
genetic endoderm, which forms precociously the anterior end of the 
notochord (o.ch.) and to the sides some of the mesoderm. For 
further details see the author’s excellent figures. : 
The essential difference between this hypothesis and that of — 
is at once seen. What the latter speaks of as endoderm cells are sa 
EMCR NOCH 
H H 
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A 
Fic. 3.—(Diagram C.) 
