1904.] 



NATURAL SCIENCES OF PHILADELPHIA, 



713 



All the forms mentioned have the primary trochoblasts in common; 

 in all, except Nereis, the gaps between the four groups of primary 

 trochoblasts are closed partly or entirely by cells derived from 2a^, 2c^ 

 and 2bS" Ischnochiton, Podarke and Dinophilus agree in that the cells 

 la*-^-^, Ib^-^-^ and Ic^-^- participate in the prototroch. In common 

 with the annelids, there is also a dorsal gap in the prototroch, owing to 

 the fact that none of the cells of the D quadrant, except the primary 

 trochoblasts, take part in its formation. 



Dinophilus differs from all the other forms in (1) the small size of 

 the primary trochoblasts, (2) that at least the posterior pair of the 

 primary trochoblasts probably divide twice meridionally, and (3) in 

 that 2a^-\ 2b^-^ and 2c^-^ also take part in closing the gaps between the 

 groups of primary trochoblasts in the quadrants A, B and C. These 

 differences, however, are of slight importance compared with the great 

 and striking similarity to the annelids manifested in the origin of the 

 cells which almost certainly form the second preoral ciliated band of 

 the adult Dinophilus. In the light of this similarity the conclusion 

 is almost unavoidable that the second preoral ciliated band of Dino- 

 philus is truly the homologue of the annelid prototroch. 



The peculiarities in the formation of the Dinophilus prototroch 

 become readily comprehensible if the character of the end result, 

 i.e., the second ciliated band, be considered, and also the time at 

 which this organ comes into functional 

 activity. The cleavages involved in the 

 formation of the prototroch are thus clearly 

 seen to be of prospective significance, or 

 inorphogenetic. 



The ciliated bands of the larval or adult 

 Dinophilus are, as compared with the pro- 

 totroch or velum of such forms as Amphi- 

 trite, Arenicola, Podarke, Ischnochiton or 

 Trochus, relatively narrow tracts (text fig. 

 VI), consisting of but a few rows of long 

 cilia, as shown in the figures of Korschelt 

 (1882) or Meyer (1887). Moreover, these 

 tracts probably do not become functional 

 until late in embryonic life. Conklin 

 (1897) has shown that the size of the pro- 



toblast of an organ is related not only to the size of that organ, but 

 also to the time at which it becomes functional. In the light of this 

 fact the small size of all the trochoblasts is easily explained. 

 4ti 



Fig. VI, head and first two 

 trunk somites of Dinophilus 

 larva, soon after hatching. 

 Drawn from a living indi- 

 vidual 



