1890.] Concrescence Theory of the Vertebrate Embryo. 707 
for a similar stage in the opossum see Selenka, 57, Taf. xvi, 
Fig. 6; in the mole, Heape, 23, Pl. xxvi., Fig. 12; in the sheep, 
Bonnet, zo, Taf. x., Figs. 39, 40. Cross sections show: the con- 
crescence of the three layers in the axis; the greater width of the 
streak in front (to this wide anterior end of the streak the term 
Hensen’s knot continues to be applied); and show also the 
increasing thickness of the streak posteriorly. The primitive 
groove, which is a shallow depression of the outer layer, appears 
first over Hensen’s knot, and thence extends gradually backward 
along the median line of the primitive streak. A transverse sec- 
tion through about the middle of the streak at this stage in the 
mole is represented in Fig. 20, and may be considered thoroughly 
typical. 
Ua Sere 
RAN) 
LONDON 
HD ^ 
X OO GrP 
RG 
; 
FIG. 20.—Section of the primitive streak of the mole; rg., primitive groove; 
Ec., éctoderm; mes., mesoderm; Zs.,entoderm; Pr., primitive streak; after Heape. 
(In sections nearer the hind end the groove Pr. does not appear; and the inner layer is 
distinct, though not separated axially from the middle layer.) 
Blastodermtic Vesicles with Primitive Streak and Head-Process.— 
In the stage we are now considering the axial thickening 
becomes sub-divided into two parts, an anterior known as the 
head-process, (Kopffortsatz), and the true primitive streak. The 
two are distinguished by the fact that the axial thickening in the 
region of the process is separated from the outer layer, but fused 
with the inner layer, while in the region of the streak it is united 
with the outer layer. At this stage we find further that, except at 
the anterior end, 7. e., in the region of Hensen’s knot, the axial 
thickening of the streak is not connected with the inner layer. 
Hence cross sections may give us three different appearances, 
according to the level at which they are taken. 
The head-process was first distinguished, so far as I am aware, 
by Kölliker, 32, also (Entw. ges., 1879, p. 271). Lieberkühn, 42, 
