708 The American Naturalist. [August, 
first showed that in it appears a small longitudinal canal, the 
walls of which form the notochord. Heape, 23, discovered that 
the hinder end of this canal opens exteriorly in the mole, and 
Bonnet, zo, made the same observation on sheep. Strahl 
describes the “ process ” in the rabbit incidentally in his paper on 
cloaca, 63 ; additional information is given by Bonnet, 77, 65—75, 
concerning the sheep, and by C. Rabl, 74, concerning the rabbit. 
Especially valuable is Fr. Carius’ dissertation, z3. In the guinea 
pig, according to Carius, after the formation of the primitive 
streak, the middle layer grows out in al directions, and lies free 
between the inner and outer layers. In front of the primitive 
streak the outgrowth takes place in three divisions, one median, 
two lateral The median outgrowth is the head-process, and it 
becomes later united with the inner layer, but at first lies entirely 
free (embryo of 13-14 days). The first indication of the forma- 
tion of a canal is an alteration of form in the cells, which elongate 
in directions at right angles to the axis of the head-process, so 
that their oval nuclei are radially placed. The change begins 
posteriorly and progresses forward; while it is going on, the 
anterior extremity of the head-process fuses with the inner layer. 
The radial cells moveapart, so that there arises a longitudinal canal ; 
subsequently the canal loses its inferior wall,sothat it becomes con- 
tinuous as a cavity with the cavity ofthe vesicle formed by the inner 
layer. In the rabbit the head-process is always free at first, but very 
early unites with the inner layer, in which condition it was found 
by Carius, 73, 18-19, at 71% days? In the rabbit Hensen's 
knot presents at this stage a small depression (the front end of the 
primitive groove), into which a small plug of tissue projects up 
from the underlying axial thickening (Carius, Fig. 7); Van Bene- 
den homologizes this with the anus of Rusconi and its plug of 
yolk matter, but inasmuch as the rabbit and bats are the only 
mammals known to have such a plug, and as theanus of Rusconi 
is necessarily at the hind end of the primitive streak, Van Bene- 
2? C. Rabl UHR sedo re INE expressly that in the rabbit the axial thickening is not 
RSEN qut den 
were probably defective ; indeed his own m un at once that the inner layer has 
been artificially separated from the overlying on 
