CERTAIN GENERAL CHARACTERISTICS 23 



much like that of a conifer with single main axis and lateral branches, ex- 

 cept that the branches lie in one plane. Faure-Fremiet (1930) has shown 

 that in Z. alternans the colony, as a whole, shows a certain degree of in- 

 dividuality in the distribution of growth and multiplication of its cells in a 

 graded order. The zooid axes of certain other branching ciliates — Carche- 

 simn, Epistylis, etc. — ^are apparently equivalent, and the growth form is 

 similar to that of a plant with a spreading crown of equivalent branches. 

 The branching hydroids and bryozoa show growth forms very similar 

 to those of multiaxiate plants and apparently resulting from similar rela- 

 tions between different axes. In certain species of Pennaria and Obelia, 

 for example, the growth form, when not altered by injury, crowding, or 

 other conditions, is in general conical like that of conifers. In many other 

 hydroids the branches are more or less equivalent as regards growth; and 

 the spreading, less regular growth form results. An acropetal gradient in 

 time of appearance of medusa buds, whether on the hydranth bodies or in 

 special zooids, occurs in many species of branching hydroids. Apparently, 

 lower levels of the multiaxiate whole arrive at the physiological condition 

 which determines formation of medusa buds earlier than higher levels. 

 Buds which develop into new individuals arise in definite graded orders on 

 stolons in certain medusae, siphonophores, and ascidians. This order re- 

 sults in gradients of size, of stage of development of the buds, and often of 

 arrangement of buds; in some forms these gradients are repeated in order- 

 ly sequence, indicating periodic or cyclical changes in condition.^ In gen- 

 eral, the character of multiaxiate growth forms is determined by the spa- 

 tial and chronological order of development of the buds which constitute 

 the axes and by the rates of growth of the dilTerent axes. All these indicate 

 the existence of graded differentials of some sort. 



PATTERN IN RELATION TO FISSION 



Fission in many of the simpler unicellular organisms is apparently 

 nothing or little more than cell division; but in many forms with definite 

 axiate pattern which reproduce by fission, a definite developmental pat- 

 tern becomes evident in the reconstitution of the parts resulting from fis- 

 sion. Brief mention of a few cases will serve to illustrate the point. Many 

 of the flagellate protozoa divide longitudinally, with cytoplasmic division 

 beginning at the anterior end and progressing posteriorly. Figures 6-9 



2 For interesting examples of these orders see Chun, 1896; Braem, 1908, for budding of 

 medusae; W. K. Brooks, 1893; M. Johnson, 1910; Ritter, 191 1, for bud orders in salpid as 

 cidians. 



