1007.] NATrKAL SCIENCES OF rilTLADEEnilA. 329 



lucres arc formed, as shown in fig. S (in this fiiiurc all the niicronieres 

 have divided or arc tlividing except the two posterior turret cells, which 

 divid(> a little later, fig. 0). The directions of these divisions and the 

 relative sizes of the daughter cells is the same as in Crepidula, and the 

 time at which the divisions occur is also the same, except that the 

 turret cells divide earlier in Fulgur than in Crepidula. 



The latest stage to which I have been able to trace with certainty 

 the lineage of every cell is shown in fig. 9. At this stage there are 

 present the following cells: 



Ectomeres -i- 



Mesentomeres 



Entomeres 7 



Total 55 



Fig. 10 shows si)indlcs in two cells of the second quartet, and when the 

 divisions here indicated are completed in the four c[uadrants the num- 

 ber of cells of known lineage is raised to 59. As a result of this work 

 on the early cleavage it may be affirmed with confidence that in Fulgur 

 the lineage of every cell up to about the 60-cell stage is itlentically 

 like that of Crepidula. 



a. The First Quartet. — Fctodermal Cross. — In Crepidula and many 

 other prosobranchs one of the most striking appearances in the early 

 history of the ectomeres is the formation of the so-caUed ''ectodermal 

 cross." This cross is composed of all the cells of the first quartet, 

 except the turret cells, and of four cells of the second quartet, one of 

 which forms the tip or terminal cell of each arm of the cross. The 

 center of the cross lies exactly at the animal pole, and one of the four 

 arms is anterior, one posterior, one right, and one left. This ectoder- 

 mal cross, composed of exactly the same cells as in Crepidula, is present 

 in Fulgur also (fig. 9 et seq.). It is not, however, so distinctly marked 

 ofT from the other micromeres as in Crepidula, and it is therefore much 

 more difficult to trace its subsequent history. In pis. XXIV, XXV, 

 figs. 8-16, the outlines of the cross are shown in heavy lines, the center 

 of the cross is marked by a short cross line, and the tip of each arm is 

 lightly stippled. In fig. 8 the cross consists of three cells in each 

 quadrant, an "apical," a "basal" and a "tip" cell (the anterior tip cell 

 is not yet formed). In fig. 9 the basal cells in the right and left arms 

 have divided, and in fig. 10 the basal cell of the anterior arm has also 

 divided. There are at this stage four apical cells and three cells in 

 each arm of the cross, except the posterior one which contains but 

 two. Later, as shown in fig. 11, the basal cell in the posterior arm also 



