No. 2.] DEVELOPMENT OF MARINE ANNELIDS. 



247 



the surface (Fig. 38). These two cells, the anlagen of the 

 paired mesoderm bands, immediately begin to elongate, and 

 soon become cylindrical in shape. The nucleus of each mi- 

 grates inward, and when it nears the middle of the cell, forms 

 a karyokinetic spindle, and a very minute cell, m, is " budded 



Fig. V. — Optical sagittal sections from camera sketches, showing invagination of the mesoderm. 

 M, mesoderm ; m, the minute cell, product of the division represented in C\ D, B, i*, ent., 

 entoderm ; X3 A'^, som. pi., somatic plate. In D the left half of prototroch is represented; 

 those cells in which the nucleoli are drawn belong to the primary prototroch, the rest are the 

 secondary prototrochal cells. 



off " ventrally. These two small cells seem to correspond to 

 those arising by the first division of the M and M in Nereis^ 

 Unio, and other forms. The peculiar thing about the division 

 in A^nphitrite is, that instead of taking place on the surface, it 

 occurs after the mesoderm cells have begun their inward migra- 

 tion, but before they are wholly inside the segmentation cavity, 

 and the spindles lie in the short diameter of the cells. Further- 

 more, the mesoderm cells, just at the time when the division 



