No. I.] 



THE EMBRYOLOGY OF CREPIDULA. 



8l 



blast cells. After the formation of the fourth quartette the 

 macromeres do not divide again until a little before the closure 

 of the blastopore, Fig. 54, and consequently the first and 

 second cleavage planes and the polar furrow serve as excellent 

 landmarks throughout a period when the egg is becoming con- 

 fusingly complex. 



In closing this section on the segregation of the layers it 

 may be well to summarize the number and position of the cells 

 in the three layers at the stage shown in Fig. 33. 



1st Quartette, Ectoblast Cells 15 



2d " " " 16 



3d " " " _^ 



39 



r Primary Mesoblasts 2 



J Mesentoblasts 2 



^ I Primary Enteroblasts .... 2 

 L Smaller Entoblasts 3 



9 



Macromeres, Larger Entoblasts 4 



Total . . . 52" 



But for the lack of one cell in the posterior arm of the 

 cross there would be ten ectoblast cells in each quadrant, and 

 this layer would be radially symmetrical. In C. adunca, as 



6 7 



Diagram 6. — Forty-two cell stage of Crepidula. Shading as in Diagrams 3 and 4. The cross 

 (shown in strong outline) lies in the position in which it was first formed. The heavy, radiating 

 lines separate the cells of the different quadrants. 



Diagram 7. — Sixty-cell stage of Crepidula. Shading and heavy lines as in the preceding. The 

 whole of the ectoblast has rotated to the left, due to the rotation of the fourth-quartette cells . 

 The " middle cells " in three arms of the cross have divided transversely. The third-quartette 

 cells on the posterior side have divided bilaterally. 



