504 The American Naturalist. [June, 
anaminota is probably not identical with the so-called primitive 
streak of the amniota, but rather with the head process as 
explained later. The fixed point of the blastodermic rim marks 
the head-end of the embryo; the parts of the ectental line, 
which grow together next behind the fixed points develop into 
the head; those a little further back into the neck, and those 
further back into the rump and tail. The parts of the circular 
rim most remote from the fixed point, d, of course con- 
cresce last. The destiny of each portion of the ectental line is 
fixed before concrescence occurs. In fact in certain cases the 
differentiation of the tissues advances to a considerable degree in 

i å N, me ullary e; ar, pion al 
on an S, = ihe bi blastodermic rio emed the Schel in German; 
5, primitive seik, bz, bl re; Yk, uncovered yolk. Compare also the text. 
the Randwulst before concrescence. This is strikingly the case 
in Elacate, in the ova of which the myotomes (or segmental divis- 
ions of the mesoderm) appear in the embryonic rim before its 
concrescence. (Ryder, 1885, 57; compare also Ryder’s obser- 
vations on Belone, 49.) 
The accompanying diagram may assist to render clear the 
process of concrescence, Fig. 2. It is intended to illustrate the 
