No. I.] THE EMBRYOLOGY OF CREPIDULA. 91 



I have found it impossible to trace the cell lineage of certain 

 parts of the cross, especially the transverse arms, farther than 

 the stage shown in Figs. 54, 56, and Diagram 11. The cells 

 become so numerous beyond this stage that I can only point 

 out the general outlines of these arms, e.g., their posterior 

 boundary and terminal cells are for a long time clearly marked 

 by the large ciliated cells which lie just behind them. Figs. 

 53> SS> 64, et seq. The anterior borders of the transverse 

 arms become confused with the lateral extensions of the ante- 

 rior arm, but even in this case it is possible for a considerable 

 time to distinguish between the anterior and transverse arms 

 by means of the large anterior turret cells or their derivatives 

 which lie in the angles between these arms. But while there 

 is a degree of uncertainty about the exact outline of the trans- 

 verse arms, there are other portions of the cross which remain 

 perfectly distinct until a period much later than any shown in 

 the figures, in fact until the larval life is practically at an end. 

 In all the later figures which show an apical view of the ^^%, 

 e.g., Figs. 50, 56, 79, the four characteristically arranged apical 

 cells can be plainly seen, while the two proximal cells in the 

 posterior arm and the median portion of the anterior arm can 

 be recognized throughout the greater part of the larval life. 

 The cells of the anterior arm which remain recognizable 

 throughout this period are, counting from the apical cells : 



Apical, outer (anterior) i 



Basals \^^''^' ^ 



( Outer 2 



Median 2 



Total .~ 



The two proximal cells of the posterior arm (id'-' and id'-^-^) 

 are recognizable for a very long period, e.g. Fig. 64. They 

 become very large, and, together with the two posterior turret 

 cells, form a large part of the head vesicle or umbrella. 



(d) Significa7ice of the Cross. — In seeking to learn the signifi- 

 cance of this peculiar structure, it will be well first of all to 

 compare it with similar structures found in the segmenting eggs 

 of other animals, then to inquire into the mechanical principles 

 involved in its formation, and finally to seek for its significance 

 in the ontogeny. 



