THE CELL ORIGIN OF THE PROTOTROCH. 12 J 



of the i6-cell stage, stippled in Fig. 8, p. 117, contain material 

 for the prototroch only, and the phenomenon in Hydroides, 

 as compared with that of other species, seems to indicate that 

 a cell in which material of a specific structure only is segregated 

 may divide once, twice, or perhaps more times. As long as all 

 the descendants of the trochoblasts enter into the formation of 

 the structure in question, it does not matter how many divisions 

 take place. This view is further supported by what has been 

 found in Capitella. According to Eisig's recent account, we 

 find in this annelid a modification of the type in the direction 

 of multiplication rather than reduction. Though sixteen cells 

 are produced from the four trochoblasts, and all contribute to 

 the prototroch, they undergo still further division before they 

 become functional. 



The first attempt to determine the exact cell-origin of the 

 prototroch was made by E. B. Wilson on the egg of Nereis. 

 He described what now seems to be a peculiar variation from 

 the type. The four trochoblasts divide as in Amphitrite into 

 sixteen cells, but four of these cells, the upper one in each 

 quadrant, do not enter into the prototroch, but the other twelve 

 cells do so. More cells are subsequently added, but their 

 origin is uncertain. If this account is correct, Nereis is the 

 only annelid in which the primary trochoblast has been shown 

 to give rise to other than prototrochal cells. 



We may now pass from a comparison of the prototroch and 

 umbrella hemispheres in those annelids which represent the 

 indirect type of development, and examine these features in 

 annelids of more direct development, in which the trochophore 

 is more or less suppressed. 



The partially suppressed trochophore of Scolecolepis is de- 

 veloped from an egg which contains an abundance of yolk and 

 is protected during its early development within the sand-tube 

 dwelling of the worm. In the 8-cell stage the four anterior cells 

 (upper quartette) are very small in comparison with the four 

 posterior ones. All eight cells divide so nearly at the same 

 time that a i6-cell stage results. Figs. 17 and 18 show that 

 the four stippled cells, which correspond to the primary trocho- 

 blasts of other forms, are far smaller than any of the other cells ; 



