1 6 2 CONKLIN. [Vol. XIII. 



three, especially after the formation of 4D. The fourth quar- 

 tette is formed just as in Crepidula, except that the cells 4A, 

 4B, and 4C are considerably larger than the macromeres, A, B, 

 C, D. There is a fifth quartette in Umbrella, the cells of 

 which are like those of Crepidula in all essential respects, 

 although showing some minor differences. The cell D divides 

 Ji7'st, and C divides soon after. Both of these divisions are 

 equal and bilateral, and the new cells formed (5C, 5D) lie pos- 

 terior to C and D. Afterward A and B divide bilaterally but 

 unequally, giving off two large cells anteriorly (5A and 5B), 

 which lie in front of A and B and on each side of 4B. Hey- 

 mons emphasizes the fact that these divisions, unlike the pre- 

 vious divisions of the entomeres, are bilateral, all the spindles 

 being parallel to the median plane. 



Afterwards the cells of the fourth quartette 4A and 4C 

 divide in the same direction as in Crepidula; but the anterior 

 product is in each case smaller than the posterior one ; 4B 

 divides much later and in a dorso-ventral direction. It will be 

 remembered that the division of this cell is dexiotropic in 

 Crepidula, and that the products afterwards lie one above the 

 other. In Umbrella this cell (4B) occupies the anterior pointed 

 portion of the embryo. 



Those cells of the fourth and fifth quartettes, which are 

 larger than the macromeres in Umbrella, are not carried 

 around to the ventral side as in Crepidula, but, on the other 

 hand, the macromeres move up through the space between 

 these cells. Virtually the same result is thus attained as in 

 Crepidula. In the latter animal the macromeres are the large 

 cells and are relatively fixed in position, while the cells of 

 the fourth and fifth quartettes are smaller and movable; in 

 Umbrella the fourth and fifth quartette cells are the larger 

 ones and are relatively fixed, while the macromeres, which are 

 much smaller, are movable. In both animals the macromeres 

 form the roof of the archenteron, while the cells of the fourth 

 and fifth quartettes form its sides. 



Two of the cells of the fifth quartette, 5C and 5D, are quite 

 small and are invaginated with the four macromeres. These 

 six cells Heymons calls the "Primare Darmzellen"; the 



