248 MEAD. [Vol. XIII. 



occurs, are squeezed in between the large somatic-plate cells on 

 the dorsal side, and the large entoderm cells on the ventral side. 

 In fact, no other cells seem to be under greater pressure at 

 any period of development, and yet the axes of the spindles 

 lie in the direction of greatest pressure (Figs. 41, 44, text 

 Fig. V, c). 



The two larger cells can be seen at the surface for a time 

 after this division, but soon disappear within the segmentation 

 cavity (Figs. 41-44, 46, 50). Once inside, they quickly assume 

 a spherical form, having the minute cells m and m still attached 

 to them (Fig. 50), and meet in the median plane. They behave 

 like teloblasts, and give rise to the two diverging rows of meso- 

 derm cells which, considerably later, break up into bands sev- 

 eral cells wide. The cells in and in can be plainly distinguished 

 until the mesoderm cell-rows contain five or six cells each 

 (Fig. 52). 



Summary. — With the attainment of the 64-cell stage the 

 rhythmical, alternating, oblique cleavage suddenly stops. Some 

 cells never divide again, some divide bilaterally, some continue 

 to divide in the alternating oblique direction. 



The formation of the apical rosette and cross is remarkably 

 similar to that in Nereis limbata} The middle cells of the 

 dorsal arms of the cross, which become the " head kidneys " in 

 Nereis, become large mucous glands in Amphitrite, while the cor- 

 responding cells of the ventral arms probably have a similar 

 destiny. The sixteen cells which are the descendants of the 

 primary trochoblasts a^'^', F"^', c^'^', (f'^', of the i6-cell stage (PI. 

 X, Fig. 11), all acquire cilia and constitute the primary proto- 

 troch (Fig. 18). They are arranged in four groups, which are 

 separated by non-ciliated areas. A little later the cells filling 

 three of these interspaces, the two lateral and the ventral ones, 

 become ciliated, so that a band of cilia surrounds the larva, 

 except in the mid-dorsal line. This completed prototroch 

 consists of twenty-five cells, — sixteen from the upper hemi- 

 sphere, and nine from the lower. Later in the development 

 the dorsal interruption is obliterated by the concrescence of 

 the prototroch cells from either side, without the addition 

 of other cells. While the interruption still persists, the 



