14 BULLETIN OF THE 



tlie formation of the body wall by the neck of the polyjjide is much 

 snitiller than Braem has suggested, and probably insignificant in amount. 

 The retreat of the points of origin of the retractor and parietovaginal (py- 

 ramidal) muscles may be in part accounted for by the normal growth in 

 area of the body wall, and in part by the actual movement of the point 

 of origin with reference to the cells of the body wall. These conclusions 

 rest upon the following circumstantial evidence. 



Owing to the small number of cells in the body wall at the tip, and 

 the comparatively slow growth of the cystid, karyokinetic figures are 

 much less frequent than in the polypide. Quite a long search has there- 

 fore not afforded cases enough to enable me to draw any perfectly satis- 

 factory conclusions as to just where, and where only, growth was taking 

 place. I have, however, seen nuclear division occurring in the elongated 

 cells of the extreme tip, rather more abundantly in the cuboidal cells 

 between the extreme tip and the gemmiparous zone, and most abun- 

 dantly in the gemmiparous zone, but here evidently having to do with 

 the origin of the polypide, muscle cells, etc. Proximal to the gemmipa- 

 rous zone, I have noted few cases of nuclear division excepting about 

 the neck of the polypide. It seems probable that the cells of the tip 

 of the branch are not to be regarded as forming a differentiated organ 

 whose elements rarely divide, but as quite capable of adding new cells to 

 the body wall. On the other hand, there is by no means a Scheitel in the 

 botanical sense, but the cells added to the body wall continue for a time 

 to divide vigorously, and finally give rise to the polypide, to the Anlage 

 of the lateral branches, and to the body wall. The cells belonging to 

 the proper cystid then cease to divide rapidly. 



I have already shown how the cells of the tip seci'ete a cuticula, which 

 becomes gradually replaced by a second cuticula secreted beneath it as 

 the body wall attains its adult dimensions. It appears as though the 

 first cuticula were secreted by the cells of the tip only. This being so, 

 since the area of the body wall increases, this first cuticula must either 

 stretch to cover the enlarged area, or else it will fail to cover it and 

 appear as isolated patches upon the body wall, and these isolated patches 

 will become more and more widely separated as the area of the body 

 wall increases. This latter condition seems to be the one realized in this 

 case. The presence of the old cuticula is easy to demonstrate, since it 

 stains deeply in hseraatoxylin ; and it may be easily distinguished from 

 that formed later, for with the same reagent this stains not at all. Figures 

 G, 11, 12, and 13 show different appearances of the cuticula at different 

 parts of the body wall. At the extreme tip (Fig. 6) there is a continu- 



