1890.] Concrescence Theory of the Vertebrate Embryo. 713 
the deckschicht remains at one end of the ovum and forms the 
träger ; it acquires an independent cavity of its own; the ecto- 
dermal portion of the plug forms a solid spherical mass, which is 
transported to the opposite pole of the ovum ; it subsequently be- 
comes hollowed out, presenting a space, which, as the later devel- 
opment shows, is the amniotic cavity. The inner layer passes 
from the edge of the träger around the sphere of ectoderm ; if 
the two parts of the plug were connected the relations of the inner 
layer would be the same as in Mus sylvatieus, Fig. 21. 
The subsequent development of the rodents with inverted 
layers is modified in various secondary features, which it will be 
unnecessary for us to study. In all typical respects the embryonic 
development agrees with that of other mammals, even as to details. 
Blastopore—The blastopore is the small opening which is 
situated at the end of the primitive streak, and leads into the 
archenteric cavity ; the portion of the archenteron, which is next 
the blastopore, is a narrow passage through the thick mass of cells 
which make the posterior part of the primitive streak; this pas- 
sage is called the blastopore canal; from its dorsal wall the cells 
arise which form the notochord. 
While concrescence is going on the blastopore ne its 
position, being always at the end of the archenteron ; after con- 
crescence is completed the archenteron expands so as to extend 
below the primitive streak, behind the blastopore; hence the blas- 
toporic canal appears as shown in Fig. 22, like a separate tube; 
it must not, however, be forgotten that it is part of the archenteron. 
The blastoporic canal remains open in  marsipobranchs, 
ganoids, amphibians, and selachians, and is well known. It is 
also found in all the amniota, but the recognition of its occur- 
rence in this group was long hindered by the fact that it does not 
exist at first as a canal. The blastopore is the opening of a tube 
through the primitive streak ; now if the cavity of this tube is 
obliterated by its walls growing together, then the primitive 
streak would become a solid mass of cells; this is the condition 
we actually find in the amniota, Fig. 22 A; since the posterior part 
of the primitive streak is morphologically the thick walls of the 
blastopore, the homologies are not altered by the temporary ob- 
Am. -Nat.—August.—2. 
