3 o 



BIOLOGICAL LECTURES. 



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. Lepidonotus, Amphi- 

 tritc, Nereis limbata, Cly- 

 menella, Nereis Dnmmerillii, 

 RJiyncJielmis, and Clepsine 

 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- 



FIG. 21. -Nereis Dummerillii (after von Wisting- responc }ino; decrease in the 

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



pole"). 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 

 Amphitrite, Clymenella, and 

 Arenicola, which represent 

 three distinct families. In 

 all three the origin of the 

 prototroch is identical, cell 

 for cell. In eight genera of 

 annelids which represent seven 

 families, tJie whole primary 



FIG. 22. -Eight-cell stage of Clef sine . Apical p ro totrOch is formed frOM 



view from drawing furnished by Dr. Whitman. * 



identical cleavage cells, the 



four so-called primary trocJioblasts of the id-cell stage (Fig. 7), 

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

 tion. These genera are Lepidonotus, Podarke, Sthenelais, 

 Hydroides, Amphitrite, Clymenella, Arenicola, and Capitella. 

 In all but two the primary prototroch consists of sixteen 



