No. 2.] DEVELOPMENT OF MARINE ANNELIDS. 30 1 



vacated, besides still occupying its original territory. The 

 manner in which this extension takes place is significant. — 

 Except for a slight early movement of the plate backward, so 

 that its edge reaches the vegetative pole in the middle line, all 

 this extension is lateral and the material moves only in the arcs 

 of small circles parallel to the prototroch or equator of the ^^^ 

 (p. 250). Thus, of course, the material just at the vegetative 

 pole does not move. The lateral edges of the plate on either 

 side come together and concresce in the mid-ventral line, and 

 separate the blastopore or stomodaeum from the posterior end. 



As a result of the metamorphosis, the entoderm and mesoderm 

 pursue their respective destinies inside the segmentation cavity, 

 while the somatic plate occupies the greater part of the area 

 of the subumbrella, including, of course, the posterior end, the 

 entire dorsal and lateral region (excepting a small tract just 

 behind the interruption of the prototroch, in the mid-dorsal line, 

 occupied by cells from the umbrella), and a large portion of the 

 ventral area: all the material lies practically in the same lati- 

 tude as at first. At this point a comparison of Amphitritc v/ith 

 Nereis is of special interest : — 



With regard to the axial relations, Wilson maintains two 

 theses upon which he bases many far-reaching interpretations. 



First, the second cleavage furrow coincides with the future 

 sagittal plane of the embryo. Since one of the resulting 

 cells is larger than the others, the embryo is one-sided in 

 the early cleavage, and the cause of the transition from the 

 spiral (oblique) to the bilateral cleavage is the reduction of this 

 posterior macromere to the size of its fellow on the opposite 

 side (Wilson,^ p. 445). Now in Amphitrite and Clymenella the 

 second furrow is inclined to the future sagittal plane so that 

 the organism may be considered bilateral from the start, and the 

 introduction of bilateral cleavage must be referred to an entirely 

 different cause (cf. p. 241). And at no time does the second 

 cleavage furrow, as a whole, even approximately separate the 

 material of the right and left sides of the embryo. Since the 

 cleavage of Nereis is essentially like that of these forms, 

 the coincidence of the second cleavage furrow with the sagittal 

 plane may well be questioned. The error seems to be in mis- 



