124 BIOLOGICAL LECTURES. 



Strengthened. If it arises sometimes from one group of cells 

 and sometimes from another, then its origin may be considered 

 either an accident of the cleavage processes which are without 

 morphological significance, or as indicating that the prototroch 

 is not homologous in the forms examined. To illustrate : let 

 us suppose that we were ignorant of the internal structure of 

 the limb in the vertebrates. This organ, being characteristic 

 of the higher vertebrate body, and having a general external 

 resemblance in different forms, would seem to be homologous 

 throughout the series. Now let us suppose that we have 

 ascertained the origin and arrangement of the component struc- 

 tures of this organ in one species man. If, on comparison 

 with other species, we find that the component structures of 

 the limb have the same arrangement and arise in the same way, 

 we may then consider these parts also to be homologous struc- 

 tures, and the homology of the entire limb is sustained thereby. 

 If, however, we find that the origin of the component struc- 

 tures is essentially different in the various forms, we may then 

 conclude either that the embryonic origin is without morpho- 

 logical significance, or that the limbs themselves are not 

 homologous. 



We will begin the comparison of the trochophore of Amphi- 

 irite with those of other annelids, assuming that the prototroch 

 cells are homologous structures, and, in keeping with the illus- 

 tration we have just given, we may expect to find : 



First, that the particular features of the cell-origin of the 

 prototroch in Amphitrite are not the result of the environmental 

 conditions which are peculiar to this Qgg, but that the same 

 origin obtains in other well-developed trochophores whose envi- 

 ronment differs considerably from that of A^npJiitrite. 



Second, that in those annelids in which the prototroch is 

 more or less suppressed and develops at a late stage, the cells 

 which we have identified as trochoblasts are small and slow in 

 dividing. 



Third, that in distantly related Ti'ochozoa which possess a 

 larval structure similar to the annelid prototroch some similari- 

 ties obtain in the cell-origin of this structure. 



Lastly, we might expect that annelid trochophores would be 



