128 BIOLOGICAL LECTURES. 



not only are they smaller, but they are the most tardy in divid- 

 ing. After all the other cells have undergone another cleavage, 

 these trochoblasts remain undivided. For this reason the 

 next stage, which is shown in Figs. 19 and 20, consists of but 

 twenty-eight cells instead of thirty-two. Three of these cells 

 correspond exactly, of course, to the secondary trochoblast 

 of Ampliitrite, Clymenella, and Arenicola, and are therefore 

 shaded as in Figs. 8 and 9, though they are very small in 

 Scolecolepis. Thus in Scolecolepis the smallness of the anterior 

 quartette of cells in the 8-cell stage, and the conspicuous 



FIG. 17. Scolecolepis, i6-cell stage, left side. FIG. 18. Scolecolepis, i6-cell stage, seen 



Primary trochoblasts stippled ; anterior hem- from anterior end. 



isphere (umbrella) bounded by dotted line. 



minuteness of the trochoblasts, as well as their tardiness in 

 dividing, correspond exactly with the small size of the umbrella 

 of the trochophore and the small size and late development of 

 the prototroch. This annelid, then, in its mode of development 

 stands intermediate between such forms as Amphitrite or Lepi- 

 donotus, which possess a typical trochophore, and Rhynchelmis 

 and Clepsine (Fig. 22), in which the development is direct and 

 the trochophore undeveloped. 



Nereis Dummerillii (von Wistinghausen) approaches even 

 nearer than Scolecolepis to the direct type. In the former the 

 anterior quartette (umbrella cells) in the 8-cell stage (Fig. 21) 

 are smaller still in proportion to the posterior quartette. With 



