4i8 PATTERNS AND PROBLEMS OF DEVELOPMENT 



usually develop hydranths at one or both ends; but after temporary ex- 

 posure to slightly inhibiting conditions that retard reconstitution but do 

 not obHterate the original polarity or the proximal end as factors in deter- 

 mination, the hydranths often show a dorsiventrality in relation to the 

 contact-free-surface differential. Tentacles may show a gradient in length 

 and rate of development decreasing toward the side in contact, and one 

 or more bases may develop from the side of the hydranth or hydranths 

 in contact (Fig. 142, F-I). 



It is sufficiently evident that development of bases or stolons, either in 

 Corymorpha or in many other hydroids, is not a specific reaction to con- 

 tact, for these parts develop under the dominance of apical regions or 

 under somewhat inhibiting external conditions quite independently of con- 

 tact. In the experiments described above, the contact is apparently mere- 

 ly a factor in determining low-gradient level. 



Aggregation of cells into masses, following dissociation by pressing 

 through bolting-cloth or otherwise, and development of individuals and 

 partial forms from the aggregates have been observed in numerous sponge 

 species and in several hydroids.'^ How developmental pattern originates 

 in these aggregates is of considerable interest. They result from chance 

 contact of cells and may differ greatly in size ; and the larger aggregates 

 may be cut into smaller ones, as desired. If an inherent persistent polarity 

 is present in the cells, orientation to each other or to some external factor 

 would have to be assumed to account for polarity of the whole. Actually, 

 however, the sponge cells chiefly concerned in the development of aggre- 

 gates do not appear to have a definite polarity, and the polarities of hy- 

 droid cells do not coincide with the polarity of the whole. If the cells 

 orient to form epitheha, their orientation must be according to their 

 polarities rather than according to a superpolarity of the whole; and if 

 they orient to some external factor, the same difficulty arises. If polarity 

 is primarily a gradient imposed on the aggregate, there are two possibili- 

 ties of origin: it may be determined by an external differential — for ex- 

 ample, an oxygen differential — or perhaps in some cases, at least in the 

 hydroids, by a chance group of cells with higher metabolism than others, 

 such as cells from the distal region of a Corymorpha stem. In sponge ag- 

 gregates canals develop in relation to a region of greater thickness between 



^' For aggregation and development from dissociated cells of sponges see H. V. Wilson, 

 1907, 1911a, b; K. Miiller, 191 1; Huxley, 1921a, b; Galtsoff, 1925; De Laubenfels, 1932. De- 

 velopment of sponges from aggregates has also been observed by the writer in a consider- 

 able number of species (unpubhshed). For hydroids see H. V. Wilson, iqiib; C. W. Hargitt, 

 1915; Child, 1928c. 



