148 CONKLIN. [Vol. XIII. 



divisions of the first quartette in the annelid are associated 

 with the more rapid differentiation of the upper hemisphere in 

 that animal. 



VI. History of the Mesomeres. 



The origin of the mesoblast was treated in a previous part of 

 this paper, and its history was traced up to the time when it is 

 completely separated from the other germinal layers. The 

 primary raesentoblast (4d) is formed at the 24-cell stage, but 

 the complete segregation of its mesoblastic and entoblastic 

 constituents does not occur until there are 65 cells present, of 

 which eight are the descendants of 4d. Of these eight cells, 

 four lie on each side of the mid line ; the two posterior ones 

 on each side are the enteroblast or intestinal cells, the two 

 anterior ones are mesoblast cells, Figs. 42, 44, 46. These 

 mesoblast cells, four in all, form the beginning of two bands, 

 which ultimately extend about halfway around the egg. 



I . The Mesoblastic Bands. 



The posterior mesoblast cell is the teloblast, or "pole cell," 

 of the bands. It is a large rounded cell, free from yolk gran- 

 ules, and, when stained, is rather darker than any of the sur- 

 rounding cells. It is frequently seen dividing, and always so 

 as to add new cells to the posterior ends of the bands. The 

 anterior cell on each side (primary mesoblast) is the first 

 purely mesoblastic cell formed. It is much smaller than the 

 teloblasts, and has less affinity for stains. These two anterior 

 cells divide soon after the teloblasts are formed. Fig. 42, usually 

 across the long axis of the bands, but sometimes in the direction 

 of that axis, Fig. 46. 



The bands grow in length both by the addition of new cells 

 at their posterior ends and by the subdivision of the cells already 

 formed. They ultimately extend around the periphery of the 

 egg from near the mid line behind to the first, or transverse, 

 furrow on each side. In all the figures up to and including 

 Fig. 53 these bands lie nearer the dorsal than the ventral side, 

 but in all stages older than Fig. 53, they are nearer the ven- 



