No. 3.] PODARKE OBSCURA VERRILL. 439 



Conklin that both have become hereditary, which would explain 

 their similarity in all these forms. 



That the two types are not sharply separated but overlap 

 is shown by the fact that the first indication of bilateral sym- 

 metry arises by a spiral cleavage, and is due to size differences 

 of cells and not to any reversed direction of cleavage. (See 

 PL XXXVII, Fig, 17.) Of value in this connection are the 

 observations recorded on p. 415, showing cases of reversion to 

 a radial type. The next divisions are bilateral (see PI. 

 XXXVIII, Fig. 27), showing divisions of 3C2 and 3d2. Hence 

 in Podarke the transition is not the sudden one described for 

 Nereis, but is gradual, as would be required by our hypothesis. 

 Mead and Conklin have already pointed out the difficulties in the 

 way of accepting Wilson's explanation of the cause for the time 

 of appearance of bilateral symmetry, and to their conclusions I 

 can add the facts in the development of Podarke, where bilat- 

 erality appears early, and no equalizing division is necessary 

 for 2d. 



The bilateral cleavages appear, as might be expected, at dif- 

 ferent times and in different ways in different animals. For 

 example, in Crepidula, Conklin has shown that the primitive 

 radial symmetry is preserved in the anterior quadrants after it 

 has disappeared in the posterior ones, e.g., the arms of the cross. 

 In Podarke it is the cross which very early shows bilateral 

 symmetry (see PI. XXXVII, Fig. 17), so that the relation of 

 bilateral cleavages solely to the processes which lead to the 

 formation of the body is not as close here as in other forms 

 {cf. Conklin, No. 5, a, p. 185). Indeed, I see no reason why 

 they should, for, as I understand it, neither the trochophore nor 

 its hypothetical ancestor, the trochozoon, has a radial, but 

 rather a bilateral organization. The eye-spots, the glands, 

 in short, all the structures of the annelid trochophore are 

 bilateral and not radial. Hence the same causes which would 

 transform the spiral cleavages into bilateral body-forming ones, 

 would operate in producing the cleavages which lead to the 

 formation of the head. What these causes are we do not 

 know, other than that they must be mainly intrinsic. 



