5l6 KNOWER. [Vol. XVI. 



proved to have certainly passed through a stage like that of 

 PI. XXIX, Fig. 10, having been selected from a lot of eggs in 

 which all stages were mixed indiscriminately. The condition 

 shown in PI. XXIX, Fig. ii, may have been reached without 

 migration by a more active multiplication of cells in the area 

 ca. from the first, the blastoderm anterior to this region remain- 

 ing comparatively inert. In other words, the center of activity 

 may have been placed more anteriorly in PI. XXIX, Fig. lo, 

 than in PI. XXIX, Fig. ii, from the start. 



In spite of this possibility of error, I believe the figures do 

 represent successive stages, and that the area ca. on the sur- 

 face of the egg in PI. XXIX, Fig. li, etc., has been established, 

 not only by a multiplication in that region, but also by the addi- 

 tion of cells migrating into it from without. This conclusion 

 seems justified by a similar examination and comparison of 

 many eggs in these stages. 



PI. XXX, Fig. 12, is a slightly older ventral surface showing 

 a like extension of the area ca., where more nuclei are now 

 found. Note especially the rather short intervals between the 

 nuclei in the posterior and lateral regions. 



PI. XXX, Figs. 13 and 14, exhibit in ventral and lateral views 

 a further result of the processes just studied. 



Comparing PI. XXX, Fig. 13, with the younger stages in 

 PI. XXIX, Fig. 1 1, and PI. XXX, Fig. 12, the number of nuclei 

 in regions anterior to the area ca. is seen to have remained 

 constant, in spite of a multiplication of cells there being 

 demonstrable. Within the former area ca. there has been a 

 great increase of nuclei, especially near the center. This is 

 undoubtedly due in part to continued cell division here ; but 

 also, as the above observation makes plain, there is evidence of 

 an addition of migrating cells resulting from multiplication in 

 more anterior regions. 



Comparing PI. XXX, Figs. 13 and 14, still closer with PI. 

 XXX, Fig. 12, additional and striking evidence is found of a 

 further migration of cells from the boundaries toward the 

 center of the former area ca. 



On rolling the ^g^ figured in PI. XXX, Fig. 12, the area ca. 

 stands out more sharply from the surrounding surface than is 



