No. I.] THE EMBRYOLOGY OF CREPIDULA. 203 



(3) The principal significance of any determinate form of 

 cleavage is prospective rather than retrospective ; almost every 

 peculiar feature of determinate cleavage can be referred directly 

 to its usefulness in building the body of the future animal. 



The cause of this determinate character of cleavage is not 

 to be found primarily in known mechanical conditions nor the 

 extrinsic factors of development, but rather in intrinsic struc- 

 tures, conditions, and forces. In the category of phenomena, 

 which at present can be explained, so far as I can see, only 

 by referring them to such intrinsic causes, may be mentioned 

 the following : (i) the dexiotropic direction of the first cleav- 

 age with the consequent alternation in direction of every suc- 

 ceeding cleavage up to an advanced stage ; (2) the reversal 

 of the usual direction of cleavage in the formation of certain 

 definite structures, or in the establishment of bilateral sym- 

 metry ; (3) the establishment of bilateral symmetry in diverse 

 directions in the different germinal layers, and the subsequent 

 coincidence of these different planes of symmetry in a common 

 plane; (4) the general phenomenon of the unequal division 

 of apparently homogeneous cells ; (5) the rapid growth and 

 slow division of certain cells {e.g., the trochoblasts) and the 

 slow growth and rapid division of other cells {e.g., the apicals) ; 

 (6) the segregation of the ectoblast into three quartettes 

 of cells, and the formation of the mesoblast in the fourth 

 quartette. 



These are but a few specific cases and many others might 

 be mentioned ; in fact, the most important phenomena of 

 development must be included in this category, — among them 

 the proper collocation of parts and coordination of results, all 

 cases of precocity and determinism, and, in fact, the ultimate 

 cause of all specific and generic characters, some of which are 

 frequently manifested at every stage from the beginning to 

 the end of development. Each and all of these phenomena 

 can at present be attributed only to intrinsic causes, since 

 known mechanical conditions are wholly unable to explain 

 them. 



The ground here taken is not one of opposition to the possi- 

 bility of a mechanical explanation of vital phenomena; such 



