No. I.] THE EMBRYOLOGY OF CREPIDULA. 85 



derived forms a linear series of cells, the apical one of which 

 lies to the right of the apical pole. The four arms thus form 

 a right-wound spiral around the apical pole. This arrange- 

 ment is especially noticeable in the nuclei of these cells, and 

 can here be recognized at the first glance. After the longitu- 

 dinal splitting of the arms and the division of the apical cells 

 to form the "rosette," this dexiotropic arrangement of the 

 arms of the cross can no longer be recognized. 



(c) Later History. — Starting from the earliest appearance 

 of the cross, when it contains twelve cells, the cell lineage of 

 the entire structure has been followed to a stage when it con- 

 tains sixty-six cells. Figs. 53, 56, and Diagram 11. The first 

 cells of the cross to divide are the basal cells in the anterior, 

 the right and the left arms (la'-^ ib'% ic'-^) in the 44-cell 

 stage, Fig. 31. In all the species save C. adunca the division 

 in the basal cell of the posterior arm is delayed until a con- 

 siderably later period. Fig. 42. By this division, which is 

 slightly dexiotropic, the basal cells are divided into a larger 

 peripheral moiety, the middle cell (la^'^-^'-id'-^-^), and a smaller 

 apical one, still called the basal cell (la'-^'-id'-^-'). Each arm 

 save the posterior contains at this stage (Figs. 32, 35, 36) 

 three cells, — a basal, middle, and terminal. 



When there are 66 cells present. Fig. 42, the basal and 

 terminal cells of the posterior arm divide, the spindle in each 

 case being parallel to the long axis of the arm. In this way 

 the posterior arm comes to be composed of four cells arranged 

 in a linear series, the two proximal (id'^' and id'-^-^) derived 

 from the basal cell and the two distal (2d''' and 2d''-^) from the 

 terminal one. Diagram 8. 



About the same time that the cells of the posterior arm 

 divide radially, the middle cell in each of the other arms divides 

 in a plane nearly transverse to its long axis into two equal por- 

 tions, the left and right tniddle cells, la'-^-^' and la'-^^^*, ib'-^-^-' 

 and Ib'■^•^•^ etc.. Fig. 42 and Diagram 7. The left moiety in 

 each arm is a little nearer the apical pole than the right, and 

 the cleavage is therefore laeotropic. This division of the 

 middle cell in the anterior, the right, and the left arms is the 

 beginning of a longitudinal cleavage of each of these arms, 



