30 DEAN. [Vol. XI. 



Segmentation. 



The first cleavage is vertical and occurs at about the same 

 interval after fertilization as in A. rutJicnns. As Salensky 

 has determined, it separates the blastomeres only in the 

 region of the germ-disc. It is deepest at the animal pole 

 but traverses the blastodisc sharply, PL III, Fig. 36, causing 

 a narrow surface fissure different from the trench-like furrow 

 that has been noted in Lepidosteus. Until the appearance 

 of the second cleavage its marginal limit is in the pigmented 

 zone of the yolk bordering the blastodisc. The present writer 

 notes that in transverse vertical section it differs little from that 

 figured in PI. II, Fig. 22 ; the furrow is more sharply cut and 

 deeper but does not penetrate into the yolk. The pigment at 

 the animal pole is now restricted to a circular area compara- 

 tively regular in outline. In the earlier stage the outline and 

 extent of the surface pigmentation has apparently no influence 

 in foretelling the direction of the first cleavage plane. 



The second cleavage, again vertical, corresponds closely with 

 that of the Gar-pike, It is approximately at right angles to 

 the first plane, exceptionally oblique, as in the figure, PI. Ill, 

 Fig. 37, and may intersect the first furrow with noteworthy 

 Polflucht, Its first outward appearance is in the region of the 

 pole, thence it may be followed as it extends marginally. 

 When it has traversed the blastodisc its depth is approximately 

 that of the first furrow, although outwardly the latter is readily 

 distinguished by its greater extent ; it now surrounds (almost) 

 the entire circumference of the Q^^g, but in the yolk region, as 

 Salensky notes, its furrow is of the most superficial character. 

 The position of the nuclei in this stage is entirely similar to 

 that of Lepidosteus, and has been figured in vertical section 

 by Salensky, Rcf. 29, PI. XV, Fig. 7: his drawing illustrates 

 the depth of the cleavage and the line of demarcation between 

 germ and yolk. A horizontal section (slightly oblique) through 

 the region of the nuclei in this stage is given in PI. IV, 

 Pig. 56; in this figure the distribution of the pigment is to be 

 traced ; at the surface it has accumulated in the rounded and 



