704 The American Naturalist. [August, 
shown by Bischoff, it lies transversely of the vesicle. The size of 
the shield is quite nearly uniform among the placented mammals 
in which it has been studied, but the size of the vesicle varies ex- 
tremely ; especially noteworthy is the excessively rapid elonga- 
Fic. 17.—Transverse section of the embryonic shield of the blastodermic vesicle of 
a sheep, en days pregnant, after Bonnet. a, outer layer of shield; 4, inner layer of 
shield. 
tion in ungulates (pig, sheep, goat, and deer) ; in the sheep, for ex- 
. ample, it trebles or sextuples its length in less than a single day 
after the shield appears. The next step is the appearance of a 
middle layer, at least in sheep (Bonnet, 70, 192-196, 17, 42), 
which shows in the fresh specimen as a slight turbidity of the 
vesicular walls just outside the edge of the shield, while in the 
region of the shield there is no middle layer whatever. Sections 
show that the new layer consists of loosely scattered cells con- 
nected by anastomosing processes. It is everywhere absolutely 
distinct from the outer layer, but merges at many pojnts with the 
inner layer. From this connection Bonnet concludes that the 
middle layer is derived from the inner layer by what must be 
called a process of delamination. So far as known to me, noth- 
ing analogous to this middle layer has yet been observed other 
mammals. The next important step, again according to Bonnet, 
10,195, is the appearance of Hensen's knot, which takes place 
while the peripheral middle layer is developing. The knot is at 
first a small thickening on the under side of the outer layer; it is 
situated on the middle line of the shield, a little nearer one end 
than the other. It is distinctly separated from the inner layer, 
but is connected with the cells of the middle layer, which have 
now developed themselves in the middle region of the shield 
also. Bonnet maintains that the knot gives off cells which con- 
tribute to the formation of the middle layer. The knot marks 
. the front end of the future primitive streak. : 
