168 TORREY 



Finally, I would direct especial attention to the complete re- 

 versions to spiral cleavage which occur with surprising fre- 

 quency. About nine per cent, of the eggs continue to divide 

 radially up to a very late period and, I believe, never develop 

 into trochophores. This seems to me an extraordinary case of 

 the persistence in ontogeny of a primitive phylogenetic stage. 



In order to facilitate comparison I have made use of the 

 system of nomenclature adopted by Tread well ('oi) (a slight 

 modification of that earlier employed by Wilson, '92, Mead, '97, 

 and Conklin, '97) in his paper on the " Cytogeny of Podarke," 

 where a full explanation will be found. In brief, the '* four 

 macromeres " are designated by the capital letters. A, B, C, D. 

 The micromeres are indicated by small letters. Each has a 

 coefficient, which indicates the generation ; and a subscript, 

 which indicates its relation to the other cells of the same gener- 

 ation. The terms " dexiotropic " (clockwise) and ** leiotropic " 

 (anti-clockwise) indicate the direction of the spiral cleavage. 

 The product of a spiral cleavage lying to the right, when viewed 

 from the animal pole, is the dextral cell ; that to the left, the 

 sinistral cell. This permits the use of the terms "right" and 

 *' left " to designate the sides of the bilaterally symmetrical body. 



It is with pleasure that I acknowledge my great indebtedness 

 to Professor E. B. Wilson for advice and criticism during the 

 progress of this research. I also wish to express my thanks to 

 Professor H. V. Wilson, Director of the U. S. F. C. Laboratory 

 at Beaufort for the facilities for collecting and preserving the 

 material that he placed at my disposal. 



Material, Methods. — The material for the following paper, was ob- 

 tained at Beaufort during the summers of 1 900 and 1 90 1 . The species 

 was first described in 1886 by Conn, who gave it the specific name 

 ^'mellita" from the fact that, almost without exception, it is found 

 in empty sand-dollar tests. Unfortunately, with the facilities at 

 hand, the animals could not be raised beyond the trochophore stage. 

 This is, in fact, a natural chapter in the life history, for the meta- 

 morphosis into the adult takes place only after an almost stationary, 

 free-swimming larval stage of long duration. 



Artificial fertilization is easily effected, although some care must 

 be exercised to prevent polyspermy. After a little experience one 



