l'.»()7.] NATURAL SCIEXCE8 OF IMII I,A DKLl'llI A. 331 



stages, it is f[uito ovidcnt tliat it is not so definite' a landmark here as 

 in Crepidula, where its cells may be identified with great certainty iij) 

 to a stage where it consists of sixty-six cells. The tvn-ret cells in 

 Crepidula grow to an enormous size and remain undivided up to a 

 stage with more than 100 cells; in FuUjur they tlo not become larger 

 than the other cells of the first quartet, and they divide twice before 

 the lOO-cell stage, as shown in figs. 13 and 16. 



h. Second and Third (Juartets of Ectomeres. — As far as I have been 

 able to follow the lineage of these cells in Fidgur, it coincides precisely 

 with that of the same quartets in Crepidula. The first subdivision of 

 the second quartet in Fulgur is shown in fig. 7 ; the first division of the 

 third quartet and the second of the second cjuartet in figs. 8, 9 and 10. 

 As a result of the divisions indicated in these figures two cells of the 

 third quartet and four of the second are formed in each c[uadrant of the 

 egg. By the divisions indicated in fig. 10 one additional second civiartet 

 cell is formed in each quadrant, and by the spindles shown in fig. 12 

 still another cell is formed, making six second ciuartet cells in each 

 quadrant of the egg shown in fig. 12. , All the third quartet cells then 

 divide, as indicated by the spindles in two of the cells in fig. 11, thus 

 giving rise to four third quartet cells in each quadrant. In figs. 12-16 

 there are six cells of the second quartet and four of the third in each 

 quadrant. Every one of these ten cells has its homologue in the 

 corresponding stages of the egg of Crepidida. 



This is as far as I have traced the lineage of all the cells of the second 

 and third quartets, though individual cells of these quartets may be 

 recognized in later stages (fig. 17). These cells of the second and 

 third quartets are in general larger than those of the first quartet, 

 which they completely surround. The third quartet cells are especi- 

 ally large, and in some cases (figs. 13, 15, 16) are quite as large as the 

 derivatives of 4d. 



In the stage shown in fig. 16 there are eighty-eight ectomeres as 



follows : 



First quartet cells 44 



Second cjuartet cells 28 



Third ciuartet cells 16 



Total 88 



5. FURTHER HISTORY OF THE MESEXTOMERES. 



The cell 4f/, formed at the 24-cell stage by la^otropic division of the 

 macromere D, undergoes dexiotropic cleavage into two equal cells, 

 4d^ and 4d- (fig. 9). At first these two cells lie to the left of the median 



