6i6 PATTERNS AND PROBLEMS OF DEVELOPMENT 



of Hypotricha represent a new pattern appearing in a certain definite re- 

 lation to the parent pattern.^ 



In Vorticella the two products of the longitudinal fission differ. One 

 remains attached to the stalk and develops peristome and gullet. In the 

 other a ciliary ring develops about the originally basal pole, the individual 

 becomes free, swims with the originally basal pole in advance, and later 

 attaches by this pole; following attachment the ciliary ring disappears, 

 and the peristomial region develops. The question whether this develop- 

 mental history involves two reversals of polarity and of dominance arises 

 from the apparently opposed character of the two patterns. If one product 

 of fission in Vorticella is less directly connected with the stalk than the 

 other and if reduction of the peristome permits development of the basal 

 ciliary ring and attainment of dominance by the basal region in this prod- 

 uct, its separation would be expected. As in the planulae of calyptoblast 

 hydroids (p. 96) and some larvae of Suctoria (p. 612), the pole function- 

 ally apical in the free-swimming stage becomes basal on attachment. 

 Both products of fission usually remain attached in the colonial vorticel- 

 lids, but certain zooids may become free-swimming "ciliospores" with 

 change in pattern and behavior very similar to that in the free form of 

 Vorticella. The free-swimming microgametes, resulting from successive 

 fissions without intervening peristome development, also show similar 

 change in pattern and behavior. Evidently this "reversal" represents a 

 characteristic form of development for this genus and, to some extent, 

 for other members of the group. 



SYMMETRIES AND ASYMMETRIES OF PROTOZOA 



A high degree of morphological differentiation and an almost endless 

 variety of symmetries and asymmetries, highly species-specific in char- 

 acter, appear in the ectoplasm of flagellate and ciliate protozoa.^ Some 

 of the varieties of larval suctorial pattern have already been described. 

 In some forms the pattern is apparently strictly radial — for example, 

 Prorodon — but the body revolves about the polar axis in locomotion like 

 free-swimming developmental stages of many invertebrates, indicating 



^ Wallengren, 1901; D. B. Young, 1922; Dembowska, 1925, 1926; Taylor, 1928. 



" Figures are not given: first, because only a very large number would give any adequate 

 survey of the variety of these morphological patterns; second, because every student 

 of zoology is familiar with some of them, and some are figured and described in every textbook 

 of zoology and many others in the works on protozoa — for example, Kent, 1880-82; Biitschli, 

 1883-89; Doflein-Reichenow, 1929; Calkins, 1933, etc.; also a voluminous literature on par- 

 ticular species of groups. 



