$ 
1886.] Physiology. ; 473 
disk of Elasmobranchs originate spontaneously. It is indeed far 
more likely that they originate by a process of segmentation in 
which the marginal cells of the blastodisk are involved the same 
as in Teleosts. Such a view is in fact supported by fig. 15 given 
in Balfour's Comparative Embryology, Vol. 11, p. 34, in which 
two free nuclear spindles are shown at the edge of the deeper- 
lying part of the blastodisk of Pristiurus in the morula condition, 
consisting of four superimposed rows of cells. Balfour’s figure 
also shows that between the lowermost cells composing the blas- 
todisk and the coarsely granular vitellus there is still a consider- 
able unsegmented stratum of finely granular plasma interposed. 
In this lower layer of finely granular plasma alone the “ free 
nuclei” are found, thus furnishing additional evidence that the 
view expressed above as to the origin of such nuclei is probably 
correct. In the disk of Raia examined by me, the cleavage 
planes are also marked by the clear margins of adjacent cells, as 
in the blastodisk of Pristiurus figured by Balfour. The blasto- 
disk of Raia here figured and described measured 1.71 millime- 
ters in width and 2.37 millimeters in length. Its thickness in the 
center was about .6 of a millimeter, and thinned out at the mar- 
Sın into a very thin layer of plasma which is obviously homolo- 
gous with the cortical or periblastic layer of the teléostean egg. 
ter stages of the blastodisk of Raia show it subdivided into 
smaller and more irregular cellular areas; the whole disk also 
again assumes much more nearly the original discoidal form 
characteristic of it previous to the beginning of segmentation. 
To Judge from the condition of the blastodisk here described, it 
of course is to be inferred that the fertilization of the egg takes 
Place while it is still in the oviduct, or possibly even before it en- 
ters the latter —Yohn A. Ryder. 3 
PHYSIOLOGY. 
GLycocenic FUNCTION OF THE Liver.—I see that in your gen- 
€ral notes on Physiology in the April number of the AMERICAN 
to be excreted as urea by the kidney. In 
cess of albuminoid over and above what is necessary for build- : 
1 This | é é. ; 
3 Am Sia artment is edited by Professor HENRY SEWALL, of Ann Arbor, Michigan. — 
: - Four, Sci., Vol. xv, P- 99, 1878. i a 
‘ 
