508 The American Naturalist. i June, 
the posterior grooved margin S; the centre of the notch, Fig. 3, 
n has remained nearly if not quite stationary, Fig. 4, pr.s., while the 
margin, s. s., of either side has been growing toward its fellow in 
the manner indicated by the arrows, and as they meet the two side 
margins grow together in the median line, making a longitudi- 
nal structure. The manner and results of the concrescence of the 
_margins from the two sides to form a median longitudinal structure, 
become clearer iri section; Fig. 4, B. The margin at the side, 
m, still shows the same relations as in Fig. 3, 8 ;. in the median 
line, however, the margins have met and intimately united, so that 
what were originally two grooves have completely united to form a 
single canal, Et, bounded above by entodermal cells, below by the 
entodermal yolk, Mz. This canal is the primitive entodermal cavity. 
A moment’s consideration renders it evident that the canal must be 
open posteriorly; this opening is the blastopore, 6/. There are 
some further details to be mentioned ; where the ectental margins 
have united in the median line, there appears a lateral outgrowth, 
mes, which is the beginning of the mesoderm; in some cases 
this mesodermic tissue appears before the margins concresce ; 
when viewed from the surface the mesoderm can be seen through 
the ectoderm, as was observed long ago; it is this faint appear- 
ance which early writers call the primitive streak, it being the 
foreshadowing of coming organization. In the middle line 
there appears a little furrow known as the primitive groove, per- 
haps the homologue of the ciliated ventral furrow of annelids. 
The Fig. 4, A, also shows ix front of the primitive streak the 
first trace, M., of the central nervous system. The blastoderm is 
seen also to be divided already into two parts, the lighter area 
pellucida, A. ., and the darker area opaca, A. o.; the latter also 
shows the first blood islands, 
From their observations, His, Kollmann and others have 
inferred that at the anterior ectental margin, a, a, a, there are pro- 
duced (from the yolk) cells, which grow in toward the embryo, 
and constitute part of the mesoderm, and are especially concerned 
in forming the first blood, which is produced always in the extra- 
embryonic area. This mesoderm of peripheral origin His has 
named parablast, a term which unfortunately has been employed 
