32 DEAN. [Vol. XI. 



furrows to the niveau of the nuclei, that the blastomeres are 

 of a more uniform texture, and that their nuclei are larger, 

 more regular in outline, and more clearly differentiated in 

 structure. 



Variations of the eight-cell stage are shown in the accom- 

 panying figure, Fig. 4, the meridional form, A and PI. Ill, Fig. 

 39, is not uncommon ; more usual variations are C and D, less 

 common are B, E and F, and forms, as G and H, showing 

 an irregular horizontal cleavage are the rarest, occurring in 

 from 4-6fo of the specimens of this stage which the writer has 

 examined. A wide range of variation in the cleavage of this 

 particular form, in view of its supposed affinities is naturally 

 suggestive, although logically perhaps, of little morphological 

 importance : for cleavage changes have been recorded in 

 Petromyzon,^ Teleosts^ and amphibians,^ and are, as yet, of 

 doubtful significance, in cases referable to mechanical causes, 

 alterations of temperature or of water density. 



In the case of the Sturgeon, however, it should be stated 

 that the variations in cleavage occurred in a general rate of 

 proportion among the normal eggs of the hatching trays, and 

 on this account appear to be worthy of especial interest. It 

 would certainly appear evident that a variation had occurred in 

 the amount of the yolk material, in cases sufficiently great to 

 permit the third cleavage plane to become horizontal. This 

 peculiarity of the Sturgeon egg will later be referred to, in the 

 discussion of the relationships of Ganoids, as suggesting a ten- 

 dency toward the evolution of a more perfect holoblastic con- 

 dition. In the Gar-pike similar variation of cleavage does not 

 occur; of all the eggs examined — to the number of several 

 hundred — no widely-marked differences from the conditions 

 figured in PI. I, Fig. 4, have been recorded. 



The variations (Figs. A-F) are disposed symmetrically with 

 reference to the first plane of cleavage (which passes from 



1 McClure, ("93) Zool. Am., XVI, p. 429. 



2 Ryder, H. V. Wilson, Agassiz and Whitman, and others. 



8 Rauber, Morph. Jahrbuch, 1S83; Jordan, J. of Morph., 1S93; v. Ebner, Fest- 

 schrift f. A. RoUett, Jena, 1S93; ^^^ results of experimental studies of Roux, 

 Morgan, and Hertwig. 



