882 The American Naturalist. [October, 
cresced blastoporic rim. The connection with the ectoderm 
renders it possible that the middle layer receives cells from the 
outer layer, but there is no direct proof of this. When the con- 
crescence is completed the mesoderm severs in the posterior axial 
region its connection with the entoderm, but retains awhile its 
connection with the outer germ-layer. The same phenomenon 
recurs in the amniota. It cannot, however, be taken to signify 
that the middle layer originates from the ectoderm, since at an 
earlier stage it is clearly entodermal. 
Mesoderm of Sauropsida—We may consider reptiles and birds 
together, since the early history of the mesoderm is very similar 
in the three classes. In dirds, the exclusively entodermic origin 
of the mesoderm is in my opinion conclusively demonstrated by 
the researches of Duval, 8, 104-117; the entoderm gradually 
thickens by migrations of its cells over a considerable axial area; 
the upper stratum of this thickened area separates off as the 
mesoderm, except that in the axial line it retains its connection 
with the entoderm; when concrescence takes place, the three 
layers are of course united in the axial line, and hence, as shown 
by Duval, the mesoderm is connected with the ectoderm. Hence 
we have two axial regions: 1°, the region of concrescence, char- 
acterized by the union of the mesoderm with the ectoderm, and 
known as the primitive streak; a little later the connection be- 
tween the mesoderm and entoderm is lost in the posterior part of 
the streak, but retained in the anterior part; 2°, in front of the 
streak the region of completed concrescence known as the head- 
process, in which the mesoderm is united with the entoderm 
only. The secondary stage is the one best known through the 
investigations of many embryologists. It forms the starting of 
Rabl’s investigations, 29, 129-140, who accordingly failed to 
recognize the true origin of the mesoderm, having mistaken a 
secondary for a primary condition. After the mesoderm is once 
separated from the entoderm, it apparently receives no further 
cells from it, except in the axial region; it is not improbable that 
along the primitive streak cells are also thrown off from the ecto- 
derm and added to the mesoderm. 
In reptiles, so far as our present unsatisfactory knowledge 
enables us to judge, the development is similar; that is to say, 
