No. I.] GAR-PIKE AND STURGEON. 33 



Jv) 



right to left in all figures), and are in this respect sufficiently 

 noteworthy to suggest that the first cleavage plane in Sturgeon 

 as in amphibian (as Roux^ recently appears to have recon- 

 firmed) determines the sagittal plane of the embryo. 



FourtJi cleavage is again vertical, and the i6-cell stage is not 

 unlike that of Lepidosteus. According to Salensky there 

 occurs in this stage in riitJiemis horizontal cleavage ; but on 

 account of the many variations which probably occur here^ as 

 well as in sturio, the writer is still inclined to believe that the 

 normal fourth cleavage^ in the sterlet may prove to* be vertical. 

 In sturio at this stage the four central blastomeres, PI. Ill, 

 Fig. 40, are irregular in outline, often projecting above the 

 surface, and are usually separated by well-marked fissures : 

 they are still, however, connected with the underlying layer of 

 the germ disc as in the earlier stage. Wide variation in the 

 i6-cell stage is to be noted in the size and shape of the four 

 central blastomeres, in the area and distinctness of the polar 

 pigmentation, and in the degree of obliquity of the fourth 

 cleavage furrow, the latter often tending to become meridional, 

 and in cases equatorial. On the lower pole of the 0:%% the first 

 and second cleavage furrows have by this time intersected. 



In the stage of fifth segmentation the writer's material is 

 deficient. 



The sixth cleavage is represented in PI. Ill, Fig. 41, and 

 may be compared with the corresponding stage of Lepidosteus 

 in PI. I, Fig. 8. Its lower pole is shown in PI. Ill, Fig. 42. 

 Cell division has by this time turned the blastodisc into a cap 

 of cells of irregular size and outline ; horizontal cleavages have 

 occurred, notably in the region of the animal pole, but these 

 cells have not been caused to be lifted above the surface of the 

 ^%^ ; meridional cleavage has occurred in the marginal cells, 

 but is expressed in an irregular manner, often by becoming 

 oblique, separating only a corner of the marginal blastomeres ; 



^ Anat. A)!z., 1S94. 



2 Thus Salensky states, " Quand la partie superieure de I'oeuf (germe) est divi- 

 see en ^//x parties — the italics are the present writer's — il apparait dans le germe 

 des sillons transversaux." 



3 The writer has been unable to compare the results of Peltsam (J?ef. 20). 



