1890.] Concrescence T heory of the Vertebrate Embryo. 503 
the discovery. Whitman, 67, 91-94, has ably defended the 
comparison made by Semper, see above. Rauber, 46, Koll- 
mann, 33, Ryder and others, 50, 57, have added to our knowl- 
edge of the phenomenon. Duval’s researches on the chick, 
z8, demonstrate concrescence there also, though the author 
appears unacquainted with the results of his predecessors. Minot, 
in the article Foetus, in Buck’s Handbook, III., 172, 173, accepts 
concrescence as the typical mode of vertebrate development. 
Concrescence in Bony Fishes—At the close of segmentation 
the germinal disc forms a cap of cells on the yolk. The disc 
(primitive blastoderm) spreads over the yolk gradually ; when it 
begins to spread its edge is 
already thickened; this thickened 
edge corresponds to the ectental 
line; the thickening is known as 
the Randwulst ; it is also called 
the blastodermic rim, which term 
Ryder and others have used. 
When the blastoderm has spread 
so as to cover perhaps a sixth 
or less of the surface, one point 
of the rim ceases to move; con- 
sequently as the expansion con- 
tinues, the edge of the disc 
bends in behind this point on 
each side, until two parts of the blastodermic rim meet, as oE 
come from opposite sides, and then grow together. This is 
illustrated by the accompanying diagram, Fig. 1; Y is the out- 
line of the yolk; 4/, the outline of the blastoderm; æ the 
fixed point; the expansion of the blastoderm has brought 
the parts 1, I, together, and they have united; the parts 
2, 2, are about to meet and unite; then 3, 3, will meet; 4, 4, 
and so on until the two halves of the ectental line are brought 
together along their entire length; their junction marks the axis 
of the future embryo, and produces a longitudinal band of thicker 
tissue, which has long been known to embryologists under the 
name of the primitive streak. The primitive streak in the 

