422 TREADWELL. [Vol. XVII. 



X2.2.2- Later, X1.1.2.1.1 and X1.1.2.1.2 divide very unequally, each 

 budding off a small cell downward, and at the same time Xj.i 

 sends a small bud upward (PI. XXXIX, Fig. 45, and PI. XL, 

 Fig. 53). These small cells, formed from X3.1 and X1.1.2.1.1, 

 have very deeply staining nuclei, and are excellent landmarks. 

 The general rotation, which all the dorsal cells undergo, affects 

 these latter cells so that the small cell, X1.1.2.1.1.2, comes to 

 lie underneath X3.2, while its sister-cell lies to its right 

 (PI. XXXIX, Fig. 48). Meanwhile the small cell, X,.2.2, has 

 migrated inward, and forms a part of the wall of the procto- 

 daeum, while its sister-cell lies at the edge of the blastopore. 

 This afterwards divides again (PI. XXXIX, Fig. 46). 



As a result of the divisions described above, the condition 

 of the X-group is as shown in PI. XXXIX, Fig. 47. At the 

 dorsal edge of the blastopore are three cells, one of which is 

 dividing. X2.2.2 divides next, unequally and in a meridional 

 direction. The smaller product lies to the outside, and later 

 X1.1.2.2.1 buds off a small cell dorsally, which lies just below 

 X,.2. These cells occupy this position for some time, but 

 subsequently divide, and the ventral edge of the X-group is 

 composed entirely of small cells. (See PI. XL, Fig. 59.) 



In PI. XXXIX, Fig. 47, X3.2 is shown very much flattened 

 and elongated, having remained undivided from the stage 

 shown in PI. XXXIX, Fig. 39. Together with the ectoderm 

 cells which have migrated from the upper hemisphere, it has 

 become excessively thin and transparent, but can always be 

 recognized by means of its large vesicular nucleus and promi- 

 nent nucleolus (PI. XL, Fig. 57). As already stated, it and 

 the "/i" cell just above it are excellent landmarks in follow- 

 ing the shiftings of position which the dorsal ectoderm cells 

 undergo. Finally A divides, and immediately afterwards 

 X3.2 divides bilaterally. PI. XL, Fig. 58, shows the spindle 

 of this division with the products of the division of /j lying 

 just above it on either side. The daughter-cells of X3.2 elon- 

 gate considerably, and push apart (PI. XL, Fig. 59). They 

 may be recognized in later stages lying in this position, one 

 on either side of and dorsal to the protodaeum. Each soon 

 buds off a small cell downward and outward. In a considerably 



