1890.] Concrescence Theory of the Vertebrate Embryo. 505 
spreading of the ectoderm (germinal disc, blastoderm, auct.) over 
the yolk, and the simultaneous formation of the primitive streak. 
The whole ovum is represented as seen in projection ; the pro- 
portions are such as have been suggested by the ova of flounders 
and frogs. Three successive stages of the expanding blasto- 
derm are represented; the first position of the embryonic rim 
(ectental line) corresponds to the dotted line, a” a’’; the concres- 
cence reaches only to the point marked 1 ; the lateral margins, 
s”, which are to concresce later, still form part of the edge of 
the blastoderm. At the next stage the ectoderm has grown very 
much, and has moved its edge to a’, a’, while the margins, s, have 
coalesced so that the primitive streak extends to 2. The exten- 
sion continues, bringing the ectental line to a, a, a, s, and carry- 
ing the primitive streak back to 3 ; behind the primitive streak a 
small area, Yk, of the Yolk is still uncovered, and corresponds to the 
so-called anus of Rusconi in frogs’ ova. The portion of the 
ectental line bounding this area differs from that which is imme- 
diately concerned in the formation of the primitive streak, s; 
although it now lies behind the primitive streak, it was previously 
in front of it, when the blastoderm covered only the minor por- 
tion of the ovum. (See s”, a” a’’.) Ultimately the yolk is entirely 
covered by the blastoderm, thus fixing the length ofthe primitive. 
streak. It is essential to notice that the blastodermic rim (ecten- 
tal line) divides into two portions, one, s, which forms. the primi- 
tive streak, and another, a”, a”, which overgrows the ovum and 
at last closes over the yolk behind the completed primitive streak. 
That the processes are essentially as described becomes evident 
upon examination of the figures given by W. His, 26, Kupffer, 
39, Coste, 74, and others. 
Underneath the entire length of the band of cells constituting 
the primitive streak is formed a cavity, which is transformed ulti- 
mately into the cavity of the alimentary canal and its appendages. 
It is termed the archenteron or entodermic canal (Urdarm). The 
mode of concrescence in elasmobranchs elucidates the formation 
of this cavity. In bony fishes the cells which form the walls of 
the archenteron lie so close together that the lumen of the canal 
