4o6 TREADWELL. [Vol. XVII. 



In orienting the embryo of Podarke, it is impossible to rely 

 on the constancy of direction of the polar furrow after the 

 64-cell stage. Even before they divide to form the fifth group 

 of micromeres, shiftings in position occur (see PI. XXXVIII, 

 Fig. 27), and immediately after this division invagination begins. 

 From this time on it is impossible to determine their position 

 with any accuracy. Evidently, then, the arrangement of the 

 ectomeres must be considered here, and that offers no excep- 

 tion to the rule formulated above. The second and fourth 

 generations of micromeres lie in the median and transverse 

 planes, the first and third lie between them. The relation of 

 the entomeres to the ectomeres, as is shown before the latter 

 shift their position, is exactly as in Amphitrite and Arenicola. 

 They lie anterior, posterior, right, and left. Owing to the 

 absence of a large D-cell, however, it is not possible to dis- 

 tinguish between the two ends of the second furrow. 



Fo7ir to Eight Cells. — The spindles of this division appear 

 about fifteen minutes after the completion of the last, and the 

 cells all divide at approximately the same time. The cells are 

 rarely in exactly the same stage at any one instant, and I tried 

 to find some indication of an acceleration of development in one 

 quadrant, such as has been described, e.g., in Unio (No. 21). 

 Such an acceleration, if present, would be of great aid in orien- 

 tation, but I do not believe that any exists. In later stages, at 

 the lower pole, the C and D quadrants divide a little more 

 rapidly than either A or B, but precisely the opposite occurs at 

 the upper pole (see PI. XXXVII, Figs. 19 and 23, showing the 

 history of the cross), and in no case was there any constant 

 difference between C and D. Neither was it possible, by any 

 staining reactions, to distinguish between the quadrants. 



The result of this division is the formation of the first group 

 of ectomeres, which lie on top of the macromeres, but are ex- 

 actly equal to them in size, or so nearly equal that their differ- 

 ences lie within the limits of error of observation (Pi. XXXVI, 

 Figs. 4 and 5). The division is dexiotropic, and it is interest- 

 ing to note that here, as in Nereis (No. 34, d, p. 387), the spin- 

 dles show an inclination from the vertical before any external 

 trace of segmentation is visible. 



