1890.] Concrescence Theory of the Vertebrate Embryo. 511 
the cavity below the blastopore. This diverticulum has been ho- 
mologized with the allantois. It is also very probably homolo- 
gous with the more nearly spherical diverticulum found in a 
similar position in teleosts, and now known as Kuppfer’s vesicle, 
from having been especially studied by C. Kuppfer, 34, 35, 
who has interpreted it as the teleostean allantois. Com- 
pare D. Schwarz, 53, 197, Taf. xii: Figs. 35, 37, etc. Around 
the blastopore is a mass of cells, continuous on the one side with 
the ectoderm, on the other with the epithelial entoderm lining the 
archenteron, and thirdly with a sheet of cells, mes, between the 
ectoderm, ec, and entoderm, ezt. 
The developmental phases just outlined seem to me to afford 
sufficient evidence of concrescence. Owing to the gradual tran- 
sition between the ectoderm (blastoderm) and the entoderm (yolk 
cells), there is no sharp ectental line as in some types. Moreover 
there is no differentiation of the tissues at the blastodermic rim, 
but only after the cells are united in the axis, hence we cannot 
distinguish parts at the periphery of the blastoderm and follow 
their union in the primitive streak as we can in certain sharks and 
bony fishes. Nevertheless we find all the essential features of 
concrescence ; the entodermal canal and the primitive streak be- 
gin at the edge of the blastoderm and grow at their posterior end 
away from the segmentation cavity and at the same rate the 
blastoderm overspreads the yolk. 
Concrescence in Sauropsida-—The early development of the 
reptilian ovum is imperfectly understood, although several valua- 
ble memoirs have been published upon it. The ova present the 
peculiarity that the posterior end of the primitive streak is a solid 
mass, the blastoporic canal being closed until a:quite advanced 
period, when it is temporarily opened. (Compare the section on 
the Blastopore.) It seems to me that the archenteron is formed 
by concrescence, in spite of the modified history of the blastoporic 
canal. Not only does the primitive merei begin its development 
at the edge of the primitive ectod d 
and grow backwards, but also the ectodermal cavity is formed 
underneath it, but there is no open blastopore so far as yet observed. 
This growth of the primitive streak and groove are very clearly 

or ger minal ] disc 

