1890.] Concrescence Theory of the Vertebrate Embryo. 619 
subsequently expands solely by its own proliferation, or whether 
it receives at its periphery accretions from the yolk-cells, is uncer- 
tain. I am inclined to think that the mesoderm does at first receive 
additions from the yolk. - In B we have a similar section, but of 
an older stage, and through the hind end of the nearly full-grown 
streak; the general arrangement is the same as in A; we note 
the following differences: the primitive streak, Pr, is very thick, 
and composed of numerous small cells, and its lateral mesodermic 
expansions, Mes, extend further around the ovum. In both sec- 
tions we see that the cells of the primitive streak are not marked 
off from those of the adjoining. entoderm. Ina longitudinal sec- 
tion, as is illustrated by that of a sturgeon, Fig. 7, ante, p. 510, 
we see that the mesoderm or tissue of the primitive streak runs 
way forward, and is thickest around the blastopore. The dispo- 
sition of the parts and the appearance of the cells vary in the 
three groups we are considering, but for our purpose it is un- 
necessary to describe these secondary differences. The points 
essential to note are that the primitive streak is formed by 
mesoderm, whichis accumulated along the line, and is thickest 
around the blastopore, and which spreads laterally between the 
ectoderm and entoderm; in the axial region the mesoderm is 
not separated from the entoderm; the blastopore passes through 
the thick hind end of the streak. 
In elasmobranchs the differentiation of the axial tissues begins 
in the embryonic rim before concrescence takes place, so that while 
the type affords peculiarly conclusive evidence of concrescence, it 
is less convenient for the study of the primitive streak since the 
hind end of the primitive streak is, as it were, divided, being con- 
tinued as the embryonic rim right and left. The degree of dif- 
ferentiation varies extremely ; in Pristiurus the mesoderm grows 
out from the embryonic rim; in Scyllium the mesoderm grows 
out, and the differentiation of the notochord begins; in Torpedo 
. (Rückert, 48, 101) the myotomes appear in the embryonic rim be- 
fore concrescence, as in Elacate among teleosts. The relations 
are further complicated by the advance in development of the 
axial structures while concrescence is going on, so that, as for in- 
a 
