MUSEUM OF COMPARATIVE ZOÖLOGY. I7 
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 from 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 0.17 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 mm., 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 
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VOL, XXII. — NO. 1. 2 
