1904.] NATURAL SCIENCES OF PHILADELPHIA. 699 



one seen which shows this division in progress. However, the subse- 

 quent position of the cells 2a^ and 2c^ shows this division to be very 

 nearly an equatorial one, though the next division of the cells 2a^ and 

 2c2 is strongly leiotropic (figs. 26 and 27). The cells 2a^ and 2c^ lie 

 above their parent cells and in contact with the stem and intermediate 

 girdle cells, at the same level with the trochoblasts. In consideration 

 of their origin and subsequent fate they will be given the name " second- 

 ary trochoblasts," which is given by Mead (1897) to cells of similar 

 origin and fate in Amphitrite and Clymenella. During these last divi- 

 sions X has given rise to a cell on its left side x^, in size similar to x*, 

 and nearly opposite that cell but at a somewhat higher level (figs. 16 

 and 18 and fig. 42). At the same time x^ is also undergoing division, 

 giving off a cell x*-^ at its lower side. This is the so-called anal cell 

 (Mead, 1897). 



(4) Segregation of Mesoblast. 



In the ovum represented in figs. 15, 16, and 17, a large spindle is 

 seen in 3D. In fig. IS the spindle is in the anaphase and the cell body 

 has elongated, clearly indicating by the unequal size of its lobes the 

 very unequal character of the division products. The latter are shown 

 in fig. 24, the division having been completed. The posterior larger 

 product is 4d, the chief mesoderm cell of mollusks and polychsetous 

 annelids. According to the law" of alternating cleavage the spindle 

 for this division should be leiotropic; in fact it is dexiotropic. This 

 reversal in direction of the spindle can be accounted for partly by the 

 crowding downward of 4d during the process of its formation. In the 

 ovum represented in fig. 16 the spindle in 3B is nearly horizontal, while 

 in that represented in fig. 18 it is inclined almost 40° to the horizontal 

 plane. This inclination is clearly brought about by the great size of 

 both 4d and X. Abutting as it does against X, 4d is prevented from 

 attaining the level of 4D, and thus the left end of the spindle comes to 

 be the lower. Why the spindle should not be inclined leiotropically 

 at first is not clear. 



The position of 4d in front of and below X and slightly to the left 

 of the mid-line is almost precisely that of 4d in Nereis (Wilson, 1892), 

 where the conditions concerned in its formation are very similar to 

 those prevailing in Dinophilus. 



4D is now reduced to about half the size of its fellows. This great 

 reduction in size of the macromere 4D is not known among the anne- 

 lids, but among mollusks it is a fairly common condition, e.g., 

 Umbrella (Heymons, 1893), Unio (Lillie, 1895), Ischnochiton (Heath, 

 1899), Dreissensia (Meisenheimer, 1901), Trochus (Robert, 1903). 



