134 H. H. NEWMAN. 



the organism, becomes the apical point of a new axis, and assumes 

 control of the entire subsequent development. For some time 

 after the new or adult axis has arisen the original lacval axes are 

 maintained in so far as the general external form and behavior of 

 the individual are concerned, but sooner or later these yield to the 

 increasing dominance of the new apical region, as may be seen by 

 the casting off or resorption of the most characteristic features of 

 the larva, such as the ciliated arms or bands, the preoral lobe with 

 its mouth and accompanying structures, and in the rudimentation 

 of the ccelomic structures of the right side. 



The developmental history of the hydroccele may briefly be 

 reviewed. Physiologically, it represents a region of rapid cell 

 proliferation in a larva which is, at the time in question, practi- 

 cally at a developmental standstill. As is well known, any 

 rapidly growing region in an organism has the capacity of domi- 

 nating the development of other regions about it and of sup- 

 pressing the differentiation of similar or homologous regions else- 

 where in the organism. It may also, in the course of time, inhibit 

 the development of rival regions and cause their atrophy or 

 resorption. Morphologically speaking, the events are as follows: 

 The hydroccele, at first a small lobe of the left anterior enteroccele, 

 buds off five primary tentacles destined to become the five radial 

 water canals. These canals assume a radiate position about a 

 central ring through which the new mouth passes. It is about 

 this ring and its five more in some species primary tentacles 

 that the new adult radial organization centers. The primary axis 

 of the adult passes through the center of the water-vascular ring. 

 This new axis can be called neither an antero-posterior axis nor an 

 axis of symmetry; it is something quite sui generis, as is recog- 

 nized by those who have given to the two poles the names "oral " 

 and "aboral." The secondary axis is also quite unique, for there 

 is no true symmetry, but merely a superficial semblance of radial 

 symmetry. 



Symmetry Relations of Larva and Adult. From the standpoint 

 of the original larval axes of symmetry, it is clear that the events 

 that lead up to metamorphosis occur typically only on the left 

 side. On this account the situation has long been thought of as 

 an outstanding example of asymmetrical development, and there 



