1890.] Concrescence Theory of the Vertebrate Embryo. 507 
shown, the division of the margin of the ectodermal disc into two 
parts, one, a, a, resting directly on the yolk, the other s, directly 
continuous with a layer of entodermal cells, B, Zz, forming a lit- 
tle groove under the margin of the disc. The two portions of 
the ectental margin have entirely distinct functions, as already 
stated: the interior, a, a, is destined to grow over and cover the 

Fic. 4.—Diagram of a on mag rd blastoderm a bo aer more advanced than Fig. 3.—4, 
surface view ; B, section along the line X, Y; te blastoderm ; a, a, a, anterior margin 
s.s., posterior margin (Sichel); 4, ya pii paca; 4.f, area ellucida ; n.r., neural 
ridges; M, neural or medullary groove; es primitive s 42, blasto greed = 
ectoderm; m, ectental margin; Ær., entodermic cells; Vi, wah mes 
s.c., segmentation cavity. 
yolk by the extra-embryonic portion of the ectoderm: the pos- 
terior, S, is destined to form the primitive streak of the embryo. 
Fig. 4 is similar to Fig. 3, but represents a more advanced 
stage. The ectodermal disk, BZ, is much enlarged, and its anterior 
grooveless margin, 4, 4, a, is relatively much more increased than 
