506 The American Naturalist. (June, 
is obliterated and does not appear until considerably later (cf. 
Balfour, Comp. Embryol., IL, 75.) . 
Concrescence in Elasmobranchs—Our knowledge rests mainly 
on the researches of His., 27, and his follower Kollmann, 
33. Fig. 3, A, is a generalized diagram of an elasmobranch 
ovum, representing the ectodermal disc, BZ as seen from 
above, resting upon the yolk, which i is not represented in the figure. 
The first change noticeable 
in the disc after the close of 
segmentation is a groove 
running completely around 
its margin, between it and 
the yolk; as the disc grows 
and expands, the groove is 
no longer present along the 
front edge, a, a, of the blas- 
ye 7 xtoderm, but only on the 
: sides and behind. About 
the same time there usually 
n ae appears a distinct notch,:%, 
- which marks the fixed point 
of the margin, and the pos- 
terior end of the disc. If, 
now, a section be made 
= across the line, xy, the 
giles 3-—Diagram of an clasmo pranch blasto- relations will be found to 
orooro.. cence view ; Bt, blastoderm; a, a be essentially as represented 
ve; m, marginal Os, line of sections ; in the diagram, Fig. 3, 8; 
ee Skee ee, oe are een en She RE 
merous nuclei; between the yolk and the ectoderm, Æe, is 
the segmentation cavity, s.c.; the groove is bounded above 
by a layer of cells, ex, which are larger than those of the 
ectoderm, and have been produced by the yolk, Mī; sometimes 
there are cells lying in the segmentation cavity at this stage, 
the formation of the mesoderm having already begun. The 
essential point to note in this stage is, as Kollmann has 


