34 DEAN. [Vol. XI. 



sometimes, as in the left of the figure, cleavages remaining 

 meridional, seem unable to penetrate the pigmented zone of 

 the yolk and thus for a time remain undifferentiated. The 

 connection between yolk and cytoplasm which thus maintains 

 seems accordingly equivalent to the condition of the marginal 

 cells in a corresponding stage of Lepidosteus, whose signifi- 

 cance has already been discussed (p. 19). In the surface view of 

 the animal pole the pigmentation, in early stages so character- 

 istic of the Sturgeon, is hardly visible, and the fissures between 

 the blastomeres are greatly reduced. The lower half of the 

 Qg^ is traversed by the third cleavage furrow, which intersects 

 the first and second furrows at moderate angles. There are 

 thus, therefore, in the 64-cell stage but 6 cells visible on the 

 lower hemisphere, a condition which differs somewhat from 

 that of r7itJieims, where, according to Salensky, "lorsque Ton 

 pent distinguer douze segments dans la partie superieure de 

 I'oeuf, sa partie inferieure n'en montre encore que 6." Nor 

 is there to be seen in the yolk region the boldly rounded 

 cell outlines which Salensky has figured, Ref. 29, PI. XV, 

 Fig. 10. 



A stage about one hour later, PI. Ill, Figs. 43 and 44, 

 already exhibits a great advance in development. The entire 

 light-colored pole is now composed of finely divided cells, even 

 to the margin of the pigmented zone ; at the animal pole the 

 outlines of the segmentation cavity may be faintly determined 

 through the fissures between the cells, and the surface of the 

 yolk-half of the egg is now seen subdivided into many-sized 

 polygonal cells. A cross section of this stage, PI. IV, Fig. 59, 

 and of the same stage at the side, Fig. 58, shows important 

 differences from the corresponding stage of Lepidosteus {cf. 

 PI. II, Fig. 28) ; the blastoderm is growing within the limits of 

 the egg's spherical curve, instead of on its surface by incre- 

 ment from below. It will be seen that the segmentation cav- 

 ity is here separated from the surface by but a single layer of 

 cells, instead of by the cell-mass of the Gar-pike ; that it is of 

 more definite size and shape, that its floor is now cellular, and 

 not of the clearly marked, merocyte-bearing, Elasmobranch 

 type of Lepidosteus. 



