88 CONKLIN. ■ [Vol. XIII. 



After this stage the cleavage at the apical pole becomes 

 more or less irregular, and is especially difficult to follow, 

 because the shape and position of the cells is so variable. In 

 fact, at about the stage shown in Figs. 53-56, the whole egg 

 becomes irregular in outline, and every part seems to be under- 

 going contraction or expansion. This, like the previous period 

 of irregularity mentioned on p. 75, and shown in Figs. 33, 34, 

 is due to divisions and changes of position which are taking 



to 



Diagrams 9 and 10. — First quartette in Crepidula, showing the later history of the 

 cross and turret cells. 



place in the entoblasts. The result of these changes is the 

 formation of the archenteric cavity; and as soon as this is 

 formed, Figs. 63, 64, the ^gg comes back to a regular form 

 again, and many landmarks which were lost for a time reappear. 

 Some marks, however, especially the cells of the anterior and 

 posterior arms of the cross, can be followed right through this 

 period. Soon after the division of the peripheral rosette cells 

 the apicals divide in a radial direction into two cells about 

 equal in size, Fig. 53 and Diagram 10. These we shall call 

 the inner and outer apicals, la'''' and ia'-''% etc. In this 

 case, as in the two preceding cleavages, the posterior cell of 

 this quartette divides first, the anterior one last. 



About the same time that the apicals are dividing, the right 

 and left basals in the transverse arms divide in a direction par- 

 allel to the long axis of the arm into the inner and outer basals, 

 Fig. 53, la'-'-^' and ia'-'-^% etc., and a little later the corre- 

 sponding cells in the anterior arm divide obliquely and unequally, 

 giving off a thin, wedge-shaped outer basal, which lies between 



