MORPHOGENESIS 805 



between the sister cells becomes very slender. They attributed this 

 behavior to dominant influence of the stalk upon the expression of cilio- 

 spore potentialities. Their investigations also included experiments 

 wherein solitary cells were dislodged from their contractile stalks. Thus 

 freed, the cells metamorphosed into typical migrants within a two-hour 

 period. Subsequent reattachment to the substrate was followed by im- 

 mediate resorption of the locomotor organelles. 



Autonomy cannot be ascribed to the stalk portion of peritrichs, despite 

 the fact that the contractile core, or spasmoneme, is composed of living 

 protoplasm. When the cell or cells are stripped from the stalk, either 

 by natural or artificial means, no further activity is manifested; it re- 

 mains inert and lifeless. Consequently, it seems unsafe to assume that the 

 stalk, or peduncle, plays more than a passive part in the differentiation 

 of the cell from which it develops. Changes in the external environ- 

 ment alone are capable of inaugurating the metamorphic processes in 

 recently affixed ex-migrants. A migrant of Zoothammum or Carchesium, 

 for example, may settle upon the substrate, metamorphose into a typical 

 trumpet-shaped cell, and secrete a section of the peduncle, then sud- 

 denly reacquire migrant characteristics and relinquish its peduncle in 

 order to establish itself in a more favorable location. 



Peritrichs of the genus Zoothamnhim present a branching type of 

 colonial organization in which the spatially separated cells show a rela- 

 tively high degree of integration. One of the large marine species, Z. 

 alternans, has proved to be especially suitable for morphogenetic studies, 

 in virtue of its development according to a definitely determined pattern, 

 as first described by Claparede and Lachmann (1858). Various aspects 

 of growth and differentiation in this form have been investigated recently 

 by Faure-Fremiet (1930) and Summers (1938a, 1938b). 



Asexual reproduction is the general rule in Z. alternans. Cells at pre- 

 determined nodes along the axis of a colony differentiate as ciliospores. 

 One by one, these mature migrants break away from the parent colony 

 and affix themselves elsewhere, to start new colonies. In this respect 

 Zoothammum resembles Vortkella. Unlike the latter, however, its 

 asexual migrants are endowed with far greater developmental potentiali- 

 ties. An affixed ciliospore loses its aboral cilia, acquires the conical vor- 

 ticellid form, and begins to elaborate the primary stalk, a process inter- 

 rupted periodically by unequal division. The successive divisions of the 



