I30 



BIOLOGICAL LECTURES. 



Fig. 21. — Nereis Ditininerillii {3.i\.e.x von Wisting- 

 hausen), 8-cell stage, from anterior end ("upper 

 pole"). 



reviewed, that when the cells of the anterior quartette are com- 

 paratively large and divide readily, the umbrella is large and 

 the trochophore active. When, on the other hand, the cells 



of the anterior quartette are 

 smaller and divide less rap- 

 idly, the umbrella is smaller 

 and the trochophore less 

 active. Lepidonotiis, AnipJii- 

 tiitc, Nereis limbata, Cly- 

 inenclla, Nereis Dnnimerillii, 

 RhyiicJiebnis, and CI ep sine 

 form a series of annelids in 

 which there is a gradual de- 

 crease in the relative size and 

 karyokinetic activity of the 

 anterior four cells and a cor- 

 responding decrease in the 

 size of the umbrella and the 

 activity of the trochophore. 



To sum up : the cell- 

 origin of the prototroch has 

 been completely worked out 

 in three annelids, viz., in 

 Aniphitrite, Clyvienella, and 

 Arenicola, which represent 

 three distinct families. In 

 all three tJie origin of the 

 pj'ototroeh is identical, cell 

 for cell. In eight genera of 

 annelids which represent seven 

 families, the zvhole primary 

 prototrocJi is formed from 

 identical cleavage cells, the 

 four so-called primary trochoblasts of the l6-cell stage (Fig. 7), 

 and all the derivatives of these four cells enter into its forma- 

 tion. These genera are Lepidonotiis, Podarke, StJicnelais, 

 Hydroides, Amphitrite, Clymenella, Arenicola, and Capitella. 

 In all but two the primary prototroch consists of sixteen 



Fig. 22. — Eight-cell stage of Clepsine . Apical 



view from drawing furnished by Dr. Whitman. 



