EMBRYOLOGY OF BDELLODRILUS 189 
the head kidney in Amphritrite and Nereis developed from the 
same cells. Conklin further states that the axial relation of 
all the blastomeres, with the possible exception of the macro- 
meres, are the same in both the annelids and molluscs, and that 
the larval mesoblast in Crepidula and Unio arises from the same 
group of ectodermal cells. 
Heath (’99) found that the prototroch in annelids and mol- 
luses was homologous, and that the twenty-two to twenty-five 
cells concerned have exactly the same origin, direction of cleavage, 
and destiny. Also that the remainder of the first quartette, 
forming the head vesicle with its rosette series and molluscan 
cross cells or intermediate girdle cells, has in all probability, the 
same fate in both. He found many other resemblances and 
concludes: 
Thus it is seen that not only in the origin and position of the various 
quartettes do resemblances appear, but that the early cleavage of these 
are in many cases cell for cell the same. In later stages close cell 
homologies cease, but the relation of the cell groups and their develop- 
ment in giving rise to larval or adult structures follow along much the 
same path. After passing these facts in review and considering the 
various structures in detail and modifications which they undergo, 
one fact presents itself with greatest clearness, that between Ischnochi- 
ton and the annelids the resemblances are moref undamental and closer 
than are the differences. 
For a more direct comparative study of Bdellodrilus with the 
annelids and molluscs, special references will be made to Clep- 
sine (Hirudinea), Dinophilus (Polychaete), and Unio (Lamelli- 
branch). In all these forms the first and second cleavages are 
meridional and divide the eggs into four unequal macromeres 
(text figs. 19-22). In Dinophilus C and D are approximately 
posterior and A and B are anterior. In the other three forms 
B is anterior, D posterior, C right and A left. In each case D 
is the largest cell; A, B and C are nearly equal; B is usually the 
smallest when variation occurs. The eight-cell stage has the 
same structure, and in all probability arises in the same manner 
in the four forms, the only apparent difference being the much 
greater relative size of the ectomeres in Dinophilus than in the 
three remaining forms. The first cleavage plane in Bdellodrilus 
