1890.] The Mesoderm and the Calom. 883 
the mesoderm arises by delamination from the entoderm, but re- 
mains connected with it along the axial line in front, że., in the 
head-process it remains connected with the entoderm only, but 
along the primitive streak it becomes secondarily connected with . 
the ectoderm. After its delamination the mesoderm expands 
independently of other germ-layers, except along the axis. That 
the relations are like those in birds has been shown clearly by 
Strahl, 39, and also by Weldon, 43, whose figure is reproduced 
(ante p.714,Fig.22, A). The intimate connection of the mesoderm 
with the entoderm at the blastodermic rim before concrescence is 
sufficiently established by Kollman, 23, 403-406, though his 
conception that this part of the mesoderm is a separate structure, 
which he terms akroblast, renders it difficult to follow certain 
parts of his description. C. K. Hofmann may also be cited, 
though his account (Bronn’s Thierreich, Reptilien, p. 1881) is of 
doubtful accuracy in several respects. L. Will, 44, 1127, finds 
that in the Gecko the mesoderm is united with the entoderm in 
the head-process, but omits to describe its exact connection with 
the primitive streak; the stages showing the origin of the meso- 
derm he does not mention. The processes involved will un- 
doubtedly be understood as soon as the concrescence of the axis 
has been worked out,—a fundamental question which as yet nota 
single investigator has heeded. 
Mesoderm of Mammals—In this class, according to the best 
recent investigations, the mesoderm appears to have a distinctly 
two-fold origin. According to Bonnet, 6, 196, part of the meso- 
derm grows out from Hensen’s knot, at a time when the knot is 
a thickening of the ectoderm, and has not yet acquired any con- 
nection with the inner layer; another portion is produced peri- 
pherally (Fig. 18 ante p. 70 5) by delamination from the inner layer; 
the two anlages grow toward one another, and unite into one 
continuous mesoderm, in which all trace of the primitive double 
origin is obliterated. Kölliker has recorded (Würzburger Fests- 
chrift) the outgrowth of the mesoderm from Hensen’s knot in the 
rabbit, and his statement has been confirmed by Fr. Carius, 7, 17. 
In later stages we find the relations of the layers similar to those 
in Sauropsida, there being a head-process with the mesoderm 
