No. I.] GAR-PIKE AND STURGEON. 37 



unpigmented cells, and the lower hemisphere of dusky color 

 and faint cell outlines. But in the later stage each of these 

 characters has become different in details: the pigmented 

 tract at the animal pole is of lighter color and of obscurer 

 boundary ; the lower margin of the white zone is now more 

 distinctly drawn and at one side is already to be recognized 

 as the dorsal lip of the blastopore ; the lower hemisphere is 

 darker in pigmentation and its marking of cell outlines is more 

 obscure. The section shows but minor changes from the con- 

 ditions of PI. IV, Fig. 61. The roof and floor of the segmen- 

 tation cavity are thicker in cell layers : the cavity itself has 

 received an angular extension in the region above the dorsal 

 lip. The latter is observed as a well marked notch between 

 the small and regular cells of the upper hemisphere and the 

 large and irregular cells of the region of the pigmented zone. 

 These now, for the first time, are to be clearly distinguished, 

 and are seen to be largest at the region of the lip and smallest 

 at the opposite side of the embryo. It is to be noted that the 

 outermost stratum of the cells of the upper hemisphere has now 

 differentiated as the ' Deckschicht.' This has been figured 

 by Salensky, although not mentioned in his text. Further 

 differences from the conditions described in riitheniis are to be 

 found in (I) the roof of the segmentation cavity ; this in nithe- 

 mis is constituted mainly of a layer of cells like those of the 

 yolk region {cf. Ref. 29, PI. XVI, Fig. 13), (II) the shape of 

 the blastopore, which in the sterlet is described as crescentic, 

 (III) the character of the blastoporic "invagination," since in 

 mthenus the lip of the blastopore appears to be widely separated 

 from the yolk cells, and (IV) the qualitative difference, as main- 

 tained by Salensky, between the cells of the upper and of the 

 lower hemispheres. The present writer would here note that 

 in his studies of stiirio, he finds nothing to warrant the qualita- 

 tive distinction that the Russian author has drawn ; he was able 

 to observe in the cells of the upper hemisphere neither zone 

 corticale of the cytoplasm nor any indications of the "petites 

 bosselures qui ressemblent a des pseudopodes lobes," and 

 believes it probable that these were due to an imperfect 

 method of preservation. 



