1890.] Concrescence Theory of the Vertebrate Embryo. 515 
formed, lined by entoderm, having cells on the dorsal, yolk on 
the ventral side; this cavity with its walls is the archenteron; 
the archenteron lengthens backwards as concrescence progresses ; 
it has, whatever its length, a small entrance (the blastopore) at its 
hind end; the blastopore is ultimately obliterated ; it is found to 
be temporarily closed in (all?) amniota during the lengthening of 
the archenteron (and primitive streak). The cells which grow 
out from the ectental line constitute the first anlage of the mid- 
dle germinal layer or mesoderm, and shining through the ecto- 
derm they produce the appearance of a whitish line, which has 
led to the name of primitive streak. The characteristics of the 
mesoderm are described in the next section. Along the line of 
junction there often appears a slight furrow in the ectoderm, 
which is known as the primitive x 
groove. j Ay 
Significance of Concrescence. It Gn Gs 
will at once be evident that if 
the process of concrescence went 
on without the actual meeting 
of the two portions of the 
ectental line the result would be 
to leave the archenteron open 
along its entire length; the bor- 
ders of the opening would be the : 
ectental line; and this line, as I Cte. 
have shown elsewhere(AMERICAN Fic. 9.—Diagram of a vertebrate embryo 
NATURALIST, 1889), corr esponds in which the lips of the cent e ape 
to the lips of the gastrula mouth; Aem. Yoik 2 joik-entoderm; +-y, gast-ula 
consequently we should have a ais. 
gastrula with an elongated mouth. This condition is illustrated 
by the accompanying diagram. It agreesin all respects with the 
gastrula type; its most noteworthy peculiarities are two :—/rsz, 
the enormous mass of yolk accumulated in the aboral portion of 
the entoderm ; second, the elongation of the gastrula or archen- 
teric cavity in a direction at right angles to the gastrula axis, Xy. 
If now the lips of the gastrula, Fig. 9, meet, and unite we 
should obtain at once the vertebrate type, cf. Fig. 4,B. Accord- 

