192 TORREY 



reversion is always to the radial from the bilateral (from the 

 type of the posterior to that of the anterior arms of the cross) 

 and not in the opposite direction." 



In Thalassenia, however, as we have seen, the step (hardly here 

 a reversion) may be from the type of the anterior to that of the 

 posterior. It seems reasonable to suppose that the radial types 

 are cases in which the primitive radial cleavage has been re- 

 tained and which have not been affected by the ''reflection " of 

 the bilateral character of the trochophore. That such a reflec- 

 tion has really taken place (Wilson, '92 ; Conklin, '97 ; Tread- 

 well, '01) and that the radial and bilateral types of cleavage are 

 distinct in origin seems to be clearly indicated by the retention 

 in some cases of the purely radial (for a fuller discussion see 

 beyond). An examination of a hundred embryos shows that 

 89 conform to the normal type, 9 to the radial, and 2 to the 

 bilateral. 



c. Equal and Unequal Cleavage. — As a result of his investi- 

 gation of the development of Unio, Lillie ('95) came to the con- 

 clusion that " the relative size of the cells in early cleavages is 

 adapted to the size and time of development of the larval organs. 

 Using this idea as a working basis, Treadwell ('99, '01) has 

 given, recently, an interesting discussion of the relations of 

 equal to unequal cleavage. The conclusions to which a study 

 of the cytogeny of Podarke led him, are fully sustained by that 

 of Thalassema. We find indications of differentiation appearing 

 just as early and in exactly the same way in both. Indeed, 

 this almost identical cleavage in these two forms and also in all 

 other representatives of the so-called '-equal" type calls for 

 more than a passing notice. On the one hand it seems evident 

 that such widely separated forms as Hydroides, Lepidonotus, 

 Podarke and Thalassema have a practically identical equal 

 cleavage because of their similarities in the trochophore stage. 

 Again, the trochophore stages are similar because all are 

 adapted to a free-swimming life of considerable duration. Such 

 a condition involves a retardation of the trunk-forming region 

 and hence the D blastomere is the same size as the others. 

 Yet it does call for an equatorial prototroch and a highly de- 



