MUSEUM OF COMPARATIVE ZOOLOGY. 17 



the neck. The change in form of the ectodermic cells from a columnar 

 to a pavement epithelium must alone cause a great increase in the ex- 

 tent of that layer. Some measurements that I have made seem to me 

 to prove that the area of the body wall does increase greatly, even out- 

 side the region whose growth Braem attributed to the addition of cells 

 from the neck of the polypide. Thus, in one case, the distance fi-om the 

 distal end of the polypide bud, which becomes the neck of the adult, 

 to the point of origin of the young retractor muscle was 0.1 7 mm. ; from 

 the same point to the septum separating the young individual from the 

 next older was 0.27 mm. In the next older individual, from the neck to 

 the origin of the retractor muscle was 0.72 mm. ; from the neck to the 

 septum was 2.0 mm. Thus assuming that the older individual passed 

 through a stage exactly equivalent to that in which we find the younger, 

 the distance from the neck to the origin of the retractors has increased 

 0.55 ram., and from the origin of the retractors to the septum 1.18 mm. 

 The first distance is that in which Braem has assumed the body wall to 

 grow by additions from the neck of the polypide, and this assumption was 

 apparently made to account for the increase in extent of this region ; but 

 the area between the origin of the retractors and the septum, which is 

 outside the region to which additions such as Braem contemplates could 

 have been made, has grown in this case very considerably more in extent. 

 This case is not a typical one, however, for we rarely find the distance 

 from the origin of the retractor to the septum to be so great. In gen- 

 eral, from observation of a number of cases, I should say that in the 

 adult the distance between the neck of the polypide and the origin of the 

 retractors, is to the distance between the latter and the septum about as 

 5 : 4, and that therefore the growth of the first region is slightly greater 

 than that of the second. From the fact, however, that the cells around 

 the neck of the polypide for a long time retain a somewhat embryonic 

 character, and may quite frequently be seen in division, this was to have 

 been expected. The conclusion which I draw from this last series of 

 conditions is, then, that it is unnecessary to suppose the addition of cells 

 from the neck of the polypide to account for the fact that the origin of 

 the retractors is carried backward from the polypide. Normal growth of 

 the body wall, such as occurs elsewhere, is quite sufficient to account 

 for it. 



To recapitulate. That portion of the cystid lying in the vicinity of 

 the neck can hardly be derived from the neck alone, for the cells still 

 show adhering to them the cuticula which they derived from the tip of 

 the branch. It is not necessary, in order to account for the movement 



VOL. XXII. NO. 1. 2 



