No. 2.] DEVELOPMENT OF MARINE ANNELIDS. 249 



undifferentiated ectoderm cells of the upper and lower 

 hemispheres are in free communication at this place, and a 

 few migrate from a position above the prototroch to one 

 below it. 



From the 64-cell stage nearly all the cells of the large dorsal 

 quadrant behave very differently from those in the other three 

 quadrants. This is especially 

 noticeable in the descendants 

 of the somatic-plate cells {d'^) 

 and the mesoderm cell {d^). 

 They do not contribute to 

 the prototroch as do those of 

 the other three quadrants, 

 and they manifest a marked 

 tendency to cleave and ar- 

 range themselves bilaterally 

 with respect to the middle 

 line of the embryo. 



The paratroch is differenti- 

 ated at a comparatively early 

 stage and is composed of four 

 cells, descendants of a" (PI. X, 

 Fig. 14), which at first lie in 

 a very slightly curved arc at 

 the posterior or dorsal lip of 

 the blastopore. A few small 

 cells, likewise descendants of d {X), also lie within this arc, 

 and, together with the paratrochal cells, mark with absolute 

 certainty the posterior end of the larva (PI. XIV). 



The mesoblast cell, d^, divides once at the surface, and the 

 two resulting cells sink into the segmentation cavity, where 

 they function as teloblasts and give rise to a pair of mesoblast 

 bands. The first division of each, however, occurs during the 

 inward migration, and the spindles lie in the short diameter of 

 the elongate cells, and apparently in the direction of the great- 

 est pressure. The fact that in other forms similar divisions 

 occur at the surface renders this cleavage especially significant 

 (text Figs. V, VI, and IX). 



Fig. VI, — Optical sagittal section of trochophore 

 at the time of the closure of the blastopore, with 

 surface view of prototroch and paratroch (par.). 

 Outline of segmentation cavity indicated by 

 dotted line ; mesoderm stippled ; m, minute 

 product of first division of paired mesoderm 

 cells ; bp, blastopore ; the prototrochal cells 

 with nucleoli are from primary, the others from 

 secondary, trochoblasts. From camera sketch. 



