34 c. M. CHILD. 



the polarity of the oocyte becomes the polarity of the early planula. 

 and second that a complete reversal of polarity occurs in the 

 course of transformation of planula into hydroid. A simple in- 

 terpretation of this reversal is possible in terms of physiological 

 gradients. The hydranth-stem axis apparently arises as a bud 

 from the original basal end of the planula. The appearance of the 

 second gradient at the basal end as the! planula elongates sug- 

 gests that the new axis arises through physiological isolation of 

 this basal region from the dominant apical region in consequence 

 of increase in length. Decrease in activity of the apical region, 

 which is indicated by the lower susceptibility of these, as com- 

 pared with earlier stages of the planula, may also decrease the 

 range of dominance and so play a part in the physiological isola- 

 tion of the basal region. In these larval stages the range of 

 dominance is undoubtedly very short because the mechanisms of 

 transmission are rudimentary (Child, '15, pp. 149-151) and 

 physiological isolation may occur at a very short distance from 

 the dominant region. In consequence of the isolation the cells 

 at the basal end become more active and lose whatever differen- 

 tiation they have attained, while the activity of other parts con- 

 tinues to decrease with the progress of differentiation, until the 

 region originally basal and least active becomes the most active 

 region of the body. With further development and with the 

 differentiation of a rudimentary nervous system the range of dom- 

 inance increases, as in the development of other forms, and 

 the hydranth which arises at the high end of the secondary gradient 

 comes to dominate the whole body, until further increase in length 

 leads sooner or later to physiological isolation again and budding 

 begins. The reversal im direction of polarity in the hydranth- 

 stem axis apparently results from the fact that in the course of 

 elongation of the planula the extreme basal region is the first part 

 to be isolated and to become more active. This precnlrmv makes 

 it the high end of the new gradient. It is evident from the 

 work of many investigators that polarity in the hydroids is ex- 

 tremely labile and readily altered by external conditions. Every 

 hydroid bud represents a new gradient and. so far as can be 

 determined, this bud which forms the first hydranth differs from 

 those which later give rise to branches only in its position at the 



