339 
the segmentation. From my own observations it may be added that, 
from the time that the embryo has attained a well-marked form, i. e. 
in an Elasmobranch embryo from the period of ZıegLer’s Fig. 7, re- 
presenting the up-rising of the medullary folds, they take no share 
whatever in building up the embryo). 
H. E. ZIEGLER ?), more than anyone else, has, in various public- 
ations, insisted on the degenerative characters presented by the mero- 
cytes. In a recent work, as well as in an earlier one, I have adopted 
his standpoint, and am now able to prove, for Scyllium and Lepi- 
dosteus, its accuracy. 
In Scyllium the merocytes appear to increase (as already known) 
only by direct cell-division until the critical period. Doubtless dur- 
ing all this time they are engaged in their task of rendering the yolk- 
plates fit for absorption by the blood. 
Before the period of 32 mm it is an easy matter to observe their 
increase by direct division. Afterwards no such signs of increase are 
apparent, and, moreover, they appear to become somewhat smaller and 
fewer. 
At the critical period, or stage of about 32 mm, a new mode of 
nutrition of the embryo is initiated 3). An internal yolk-sac of one thin 
layer of cells (hypoblastic) is formed, and the yolk is gradually drawn 
into this sac, and thence into the alimentary canal. In Scyllium this 
yolk is not, as in earlier stages, already emulsified or prepared for 
digestion and absorption. It consists of unaltered yolk-plates, and 
there are no merocytes among them. Then, or very soon 
afterwards — certainly in embryos of 4 cm is this the case — the 
pancreas comes into activity *), and in this way digestive fluids are 
1) The figure in Herrwie’s Embryologie, Aufl. V, fig. 129 (from 
Rickert) might be used equally well to prove the wandering of the mero- 
cytes into the yolk as their migration from the yolk to form cells of the 
embryo. Horrmann’s observations to the same end only prove that in 
Teleostei large cells (megasphaeres of Rickert) occur within the embryo, 
as was already known from the work of Rückerr and ZieeLer. It is a pure 
assumption that the cell near the notochord figured by Horrmann (Z. W. 
Z., Bd. 46, Fig. 19, Pl. 35) had a merocytic history and origin. | 
2) H. Ernst Zirerer, Die Entstehung des Blutes bei Knochenfisch- 
embryonen. Arch. f. mikrosk. Anat., Bd. XXX, p. 596—665, 2 plates. 
— Die biologische Bedeutung der amitotischen (directen) Kernteilung im 
Tierreich. Biol. Centralbl., Bd. XI, p. 372—389; and in other papers. 
3) From this period onwards the yolk-sac circulation, as a means of 
nutrition of the embryo, loses gradually all its importance. 
4) This is rendered very evident by the numerous zymogen granules 
seen in preparations stained with methyl-blue-eosin. 
24 * 
