1890.] Concrescence Theory of the Vertebrate Embryo. 623 
remains a narrow fissure between the cellular entoderm, Ent, and 
the yolk; the archenteron communicates with the exterior by 
the blastopore, M. The entoderm is a loosely put together 
stratum of ¢ells, which passes over anteriorly into a ridge of the 
yolk in which cells are being produced around the already 
accumulated nuclei; this ridge, £ zv, is the germinal wall. Poste- 
riorly the cell layers are much thicker, A ; the ectoderm is clearly 
differentiated from the underlying cells, which are all more or 
less alike, though they represent both the entoderm and meso- 
derm. From this connection and from the fact that the con- 
nection between the ectoderm and mesoderm, which is so well 
known to exist after the primitive streak has attained its full 
2 SUN ERE 
NE i N 
FIG. 14.—Transverse sections = a germinative area with half formed primitive streak ; 
after Duval. e ara ge the anterior region of the area pellucida; 2, through the prim- 
ea streak; C, part of A en eed. Ec, ectoderm; Mes, mesoderm : ; Ent, entoderm ; 
w, germinal wall — Ach, archenteric cavity ; Pr, primitive groove; Pi, yolk. 
length, Duval concludes that the mesoderm arises primitively 
from the entoderm. Transverse sections afford additional informa- 
tion. The accompanying figure, 14, represents cross sections of 
a germinal area the primitive streak of which has attained about 
one-half its full length. The first section, Fig. 14, A, passes 
through the anterior region of the area pellucida; it shows the 
large cavern, Ach, of the archenteron hollowed out in the yolk; 
the entoderm, £z, above the cavity is a thin layer of cells, con- 
nected laterally with a projecting shelf of yolk, Zw (the dourrelet 
entodermo-vitellin of Duval), which is rich in nuclei, and subse- 
ps 
«ud 
* 
