438 TREADWELL. [Vol. XVI I. 



Part II. General Considerations. 



Axial Relations. 



The axial relations of the first cleavage planes have been 

 described above (p. 405). Those of the trochophore agree with 

 Amphitrite. The mesoblast bands occupy from the first their 

 definitive position at right angles to the prototroch. Although 

 the X-cells, as in Nereis, occupy a final position much farther 

 from the prototroch than they were at first, the cells between 

 them and the prototroch have a very different origin from the 

 cells in a similar position in Nereis, and I have seen no evi- 

 dence for a shifting of the neural axis, such as has been 

 described by Wilson (No. 34, d). The descendants of the 

 X3.2 group form a row of large cells across the body which 

 look like teloblasts, but I have no evidence that they really 

 have that function. (See p. 423.) I do not think that all of the 

 ventral ectoderm of the trochophore arises from the X-cells. 

 All the evidence indicates that some of the other second, and 

 probably to a limited extent the third, quartette cells have this 

 fate. (See PI. XXXIX, Fig. 46.) 



Types of Cleavage. 



In Podarke, as in other annelids, and in gasteropods and 

 lamellibranchs, two types of cleavage, the radial and the bilat- 

 eral, have been distinguished, the former appearing first, and 

 being gradually displaced by the latter as development proceeds. 

 The subject has been thoroughly discussed by Wilson (No. 

 34, d), Mead (No. 22), and Conklin (Nos. 5, a and 5, b), and I 

 need not go over the ground again. As I have said in another 

 place (see p. 470), the most reasonable explanation of these 

 cleavages to my mind is that they were primarily mechanical, 

 and that morphogenetic processes have been secondarily moulded 

 upon the primitive forms. The first of these causes leads to 

 the radial or spiral divisions which appear first, and the second 

 to the bilateral divisions which have a direct reference to 

 the bilaterality of the future body. I agree perfectly with 



