6o 



CONKLIN. 



[Vol. XIII. 



4. Formatio7t of the Third and Last Quartette of Micromeres 

 and Complete Segregation of the Ectoblast. Figs. //- 

 ig. Diagram 4. 



The last quartette of ectomeres is formed by dexiotropic 

 cleavage. The axis of each spindle lies transverse to the 

 median plane of each macromere, and nearer the right side 

 than the left. The right end of the spindle is higher than 

 the left, and lies on the right side of the macromere near the 

 furrow between contiguous spheres, and in the space between 

 successive micromeres of the second quartette. The left and 



Diagram 3. — Crepidula, twelve-cell stage. 



Diagram 4. — Crepidula, twenty-five cells ; i, turret cells (trochoblasts). In these and some of the 

 following diagrams the macromeres and first quartette are unshaded ; the second quartette is 

 stippled ; the third quartette is shaded with lines ; and the fourth quartette (4d) with dots 

 and circles. The direction of the various cleavages is shown by means of arrows. 



lower end of the spindle lies near the mid line of the macro- 

 mere, and usually beneath the second group of micromeres, 

 Fig. 17. As division advances, the right and upper end of 

 each spindle is lifted to a higher level and swung inward toward 

 the apical pole. When the cells of this quartette are separated 

 they do not rotate in a clockwise direction, carrying the whole 

 plate of micromeres with them, as in the formation of the first 

 and second quartettes, but the ectoblastic plate remains fixed 

 and the third group of micromeres is merely pressed into the 

 spaces between the micromeres of the second group. Figs. i8, 

 19, 20 and Diagram 4. 



Into these three quartettes of micromeres is gathered the 

 entire ectoblast of the developing embryo ; and from these 



