DIFFERENTIATION IN HYDROID COLONIES. 

 II. AGLAOPHENIA. 



H. B. TORREY. 



In a former paper ^ I called attention to the fact that many- 

 species of hydroids change their structural type with age, that is, 

 in the direction of their growth. "Stems straight proximally 

 may become sinuous distally. Branches which alternate during 

 the early stages of colonial development may later originate in 

 pairs. Length and annulation of hydrothecal pedicels, size, pro- 

 portions and ornamentation of hydrothecae may similarly vary 

 with the distance from the base of stem or branch." An attempt 

 to analyze the problems in differentiation suggested by these facts 

 was begun, upon a species of campanularian peculiarly well 

 adapted for experimentation. The results reached favored the 

 view that the progressive changes in the differentiation of the indi- 

 viduals of a colony are not dependent upon changes of external 

 conditions, but upon changes of internal conditions ; and that 

 these changes are of a physiological character, and do not lead to 

 a conception of morphological deter?niiiants or residual germ plasm. 



The bearing of the facts upon the problem of senescence was 

 also suggested. For, on the stem of such a hydroid as Cain- 

 panularia — with a cymose type of budding — the individuals 

 appear as successive generations, forming, at the same time, parts 

 of the whole colony. Accordingly, a comparison of any two 

 individuals might be expected to show a difference in their struc- 

 ture corresponding in some degree to a period of physiological 

 change in the colony equal to their difference in age. 



Since the publication of these results, several important papers 

 have appeared expressing radically different views on the relation 

 of natural death and development. Minot {Pop. Sc. Mo., Nov., 

 1907, p. 472) holds that as cells differentiate they lose something 

 of their capacity to live. Loeb {Pfiuger' s Arch., 124, 1908, p. 

 411), after stating this conclusion in chemical terms as follows : 



^Univ. Cal. Publ., Zoology, 2, p. 323. 



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