CELL DIVISION IN EGGS OF CREPIDULA. 513 



the "micromeres" 1A 1 , IB 1 are very large and contain yolk; in the macromer- 



I A 2 the nucleus and cytoplasm lie on the animal pole side of the cell on the 



other hand IB 2 is entirely cut off from the animal pole. A study of the later 

 cleavages of such eggs after pressure has been removed shows that only those 

 cells which preserve a cytoplasmic area on the animal pole side give rise to 

 micromeres; therefore, IB 2 being cut off from the upper half of the egg is incapable 

 of forming micromeres and is in the condition of the cell 1 D* in figs. 23 and 24 . 

 Figs. 53-55 are 8-cell stages in which some of the "micromeres" are much larger 

 than usual and contain yolk. It is interesting to observe that all such "micro- 

 meres" are dividing synchronously with the macromeres and in general in the 

 same direction as the macromere from which they were derived. In fig, 53, the 

 "micromeres" 1A 1 , IB 1 , ID 1 are dividing in a direction similar to that of the 

 normal micromere (see fig. 56, lc) but the spindle is so placed in each cell that tin 

 peripheral portion instead of being a small "turret" cell will be a large ; oik- 

 containing cell; the position of the spindles in the macromeres IB 2 -] I) 1 \g Ino- 

 tropic, which is characteristic of the fourth cleavage (formation of second quartet) 

 but in 1A 2 the position of the spindle is dexiotropic. In fig. 54, the normal 

 position of the spindles is reversed in 1A 1 , and 1A 2 . In fig. 55 the spindle posi- 

 tions are all dexiotropic, the reverse of normal, though the position of oentrosome 

 and nucleus in IB indicates that the direction of cleavage in this cell will be 



laeotropic. 



In fig. 76 is shown an egg in which pressure in the direction of the chief axis 



of the egg, during the third cleavage, led to the formation of eijdit macromeres, 

 each of which has now given off one micromere; the direction of division at the 

 third cleavage was prevailingly laeotropic, at the fourth cleavage dexiotropic in 

 quadrants, C and D, and prevailingly laeotropic in A and B. Fig. 77 shows 

 another egg pressed in the direction of the chief axis after the formation of the 

 first quartet, and during the formation of the second and third quartets; the 

 abnormalities in the latter are indicated by the lettering and arrows l>etween cells. 

 Fig. 78 represents an egg which was pressed between graphite-plate electrodes for 

 two minutes, during which time a current of 5 mil. amp. was passed between the 

 electrodes, the egg was then left in sea water under normal conditions for 22 

 hours; there are six macromeres and twelve micromeres; four of the macromeres 

 and three of the micromeres are dividing, the stage corresponding to that of the 

 formation of the second set of micromeres in normal eggs. The pole at which 

 the micromeres form differs in different blastomeres and is widely different from 

 that of normal eggs. Fig. 79 shows an egg which was pressed for ten minutes 

 between graphite-plate electrodes between which a current of 5 mil. amp. was 

 passed, the egg was then left in normal sea water for 16 hours. The second set 

 of micromeres are being formed and the first set are dividing, but the egg shows 

 plainly the effects of pressure in the direction of the chief axis. 



The egg shown in fig. 56 was evidently pressed after the formation of the 

 first set of micromeres (la-Id) which are quite normal; on the other hand, the 



84 JOURN. ACAD. NAT. SCI. PHILA. VOL. XV. 



