288 WATASE. [Vol. IV. 



as in (4-2') Fig. 27. The corresponding segment on the right 

 side (4-2), Fig, 28, tapers gradually to a point as it comes 

 nearer to the periphery of the disc, and does not expand as in 

 (4-2) marginal segment in Fig. 27. In my former communica- 

 tion ^ I failed to trace the true homology of that diminutive 

 marginal segment in Fig. 28 (4-2') and (4-2), and a certain 

 confusion has resulted in the naming of the marginal cleavage 

 furrows. 



Fig. 29 shows the blastoderm in the same stage as Fig. 28. 

 The arrangement of segments is pretty much like that in Fig. 

 27. An interesting point in this specimen is that there exists 

 a pair of triangular areas in the anterior half of the blastoderm, 

 where the cytoplasmic cleavage of the segments is not complete. 

 I cannot tell whether this is due to the incomplete division or 

 due to the refusion of the original segments once completely 

 divided. At any rate, it is interesting to notice that the dis- 

 tribution of these areas of incomplete division is perfectly 

 bilateral and can be traced originally to a pair of single seg- 

 ments in the anterior half of the blastoderm in the eight cell 

 stage (Fig. 22, PI. XI). 



The faint cell-boundaries, such as are represented in the 

 figure, were all that could be seen soon after the blastoderm 

 was prepared fourteen months ago. The specimen is still in 

 existence, but fusion of the segments is almost complete and 

 the original faint boundary lines are hardly discernible. The 

 area looks, in fact, like an unusually large marginal segment 

 with four caryokinetic figures in it. 



The remaining three stages. Figs. 30, 31, and 32, do not need 

 any special comment. The condition of the nucleus in every 

 cell has been faithfully represented, showing at a glance which 

 segment is further advanced than the others. 



Fig. 30 represents a stage with 60 segments — 30 on each side 

 of the median axis or the plane of the first cleavage furrow. 

 Counted with reference to the plane of the second cleavage 

 furrow, we find 32 segments in the front half of the original 

 blastoderm, and 28 in the posterior, showing that growth in 

 the anterior half is more vigorous than in the posterior. In 

 other words, each of two segments in the anterior half of 

 the blastoderm in the 4 cell stage, such as are shown in Figs. 



1 Johns Hopkins University Circulars. March, 1889. 



