THE SPIRAL TYPE OF CLEAVAGE. 243 



The simplest explanation would seem to be that the energy of 

 the egg is used where most needed. In cases where the larva 

 swims at a very early stage, the cilia appear correspondingly 

 early, but in other cases the trochoblasts remain apparently at 

 rest, perhaps for a long time, till some time before they are 

 needed the differentiation appears. To me this appears to be 

 simply a case of the saving of energy. The energy of the egg 

 is so exactly distributed that none is wasted in the development 

 of organs before they are needed. The distribution of the 

 energy must be connected with some physical or chemical 

 change, and in this way intrinsic differences may arise between 

 the various cells, but the differences between the quiescent 

 trochoblasts and the other cells do not necessarily signify 

 that the former contain a special substance which makes them 

 distinctively trochoblasts from the time of their formation. Of 

 course at some time they do become distinctively trochoblasts, 

 but simply because of their relation to the whole. 



Again, if the course of the cleavage in different forms be 

 followed, the different time relations in the division of the 

 various cells indicate the nicest adjustment to prevailing con- 

 ditions. I cannot see how such adaptations could arise, nor 

 how others can arise in the future without the closest relation 

 between the parts. 



The close relation of many cells to their environment, even 

 after differentiation, is evident from many of the facts of regen- 

 eration, and experiments on the early developmental stages of 

 many eggs lead us to the same conclusion. In those cases 

 which seem to point to a directly opposite conclusion, it is 

 sometimes perfectly clear that the apparent independence of 

 the cell or cells in question is really due to differentiation, but 

 there is also reason to believe that in many cases the essential 

 features of the environment are not recognized, and that what 

 appears to be independence is in reality dependence upon 

 factors of the environment whose existence is unsuspected. 



It is difficult to understand how "precocious segregation" 

 could have appeared at all without a very intimate interrelation 

 of parts ; but, on the other hand, it seems clear that the exist- 

 ence of a high degree of continuity would favor it. Once estab- 



