28o MEAD. [Vol. XIII. 



stage, consisting of four smaller cells at the animal pole alter- 

 nating with four larger ones at the vegetative pole. The destiny 

 of these cells in all well-ascertained cases is the same; the four 

 smaller ones together give rise to the umbrellar region of the 

 trochophore, the four lower cells to the subumbrella. When 

 the cells of the upper quartette are comparatively large and 

 divide readily, the umbrella is large and the trochophore active. 

 When, on the other hand, the cells of the upper quartette are 

 smaller and divide less rapidly, the umbrella is smaller and the 

 trochophore less active. Lepidonotus, Amphitrite, Nereis lim- 

 bata, Clymenella, Nereis Dummerillii, Rhynchehnis, and Clepsine 

 form a series of annelids in which there is a gradual decrease in 

 the relative size and karyokinetic activity of the four upper 

 cells, and a corresponding decrease in the size of the umbrella 

 and the activity of the trochophore. The molluscs might be 

 arranged in a similar series, e.g.^ Patella, Crepidula, Unio, and 

 Umbrella. 



In the i6-cell stage the ultimate destiny of four cells, those of 

 the second quartette from the animal pole (trochoblasts), has 

 been completely worked out in Amphitrite and Clymenella. 

 These give rise to the primary prototroch in exactly the same 

 manner in both forms.* Though the worms belong to rather 

 distantly related families (Maldanidae and Terebellidae), and the 

 eggs are different in size, in the quantity and quality of yolk, 

 and in the relative size of the trochoblasts, yet the latter agree 

 perfectly in mode of origin and in destiny, and must be placed 

 in the category of homologous cells. The observations in other 

 forms, so far as they have been extended, bear out this homol- 

 ogy. — In Lepidonotus the divisions of the trochoblasts take place 

 exactly as in AmpJiitrite and Clymenella, and when they are 

 completed the trochophore almost immediately begins to swim. 

 The prototrochal cilia are produced, in part at least, by these 

 cells, and a complete correspondence is very probable. 



Scolecolepis contrasts sharply with Lepidonotus, since it has 

 an almost completely suppressed trochophore. The upper hemi- 

 sphere, umbrella, is itself very small, while the trochoblasts (.-*) 



* These two annelids are the only ones in which the history of all the products 

 of the trochoblasts has been completely worked out. 



