No. I.] THE EMBRYOLOGY OF CREPIDULA. 83 



cells divides in a dexiotropic direction, and in a plane at right 

 angles to that of the previous cleavage. When the division is 

 completed it can be seen that the outer daughter cells are 

 largely overlapped by the apical or stem (la'-'-id'-') cells. 

 The outer cells (la'-^-id'-^) become the basal cells in the arms 

 of a cross, the origin, history, and significance of which will 

 now be considered. 



I. The EctoblasHc Cross. 



In the history of this ectoblastic cross, and of the four turret 

 cells which lie between its arms, is comprised the history of the 

 whole upper hemisphere of the larva. It is the one landmark 

 which has made it possible to follow the cell lineage in some 

 cases to the formation of definitive organs. In treating, then, 

 the history of the first quartette, I shall first deal with the 

 cross, and then take up the turret cells. 



(a) Formation. — Although two-thirds of all the cells enter- 

 ing into the cross are present and in position at the stages 

 shown in Figs. 23-25, the cross itself does not become 

 apparent until the four small cells have been formed which 

 become the tip cells of its arms. Fig. 29, 2a''-2d'', and 

 Diagram 6. These tip cells come from the second quartette, 

 though all the other cells of the cross belong to the first ; 

 for convenience, however, I shall here treat of the cross 

 as a whole, though its tip cells would properly come under 

 the section on the history of the second quartette of ecto- 

 meres. The tip cells are formed by an oblique and un- 

 equal division of four of the belt cells, 2ai-2di, the cleavage 

 being distinctly laeotropic. The upper and smaller moiety 

 becomes the terminal or tip cell (2a'-'-2d'') in each arm of the 

 cross, though its relation to the other cross cells is so close 

 that I doubt whether any one who had not watched its forma- 

 tion would suspect that it was not derived from the basal cells 

 of that structure. The cross then contains all the cells of the 

 first quartette except the turret cells, and in addition the tip 

 cells, which come from the second quartette. When it first 

 appears it consists of twelve cells ; the four apical cells form its 

 centre, while there are two cells in each arm, one basal, the other 



