ILLUSTRATIONS xxiii 



165. Group formation in Chlamydonionas, showing groups 



formed in a mixture of cells differing in sex 674 



166. The mating reaction in Pi^;w;2^r/V/w ^//rj,^;74 . . . . . 691 



167. Conjugation in Cycloposthium hipalmatiim showing hetero- 



morphic pronuclei 702 



168. Dif^ugia corona; members of four different clones, showing 



diversities in characteristics 724 



169. Polytoma uvella and P. pascheri; the four races used in the 



breeding experiments of Moewus 733 



170. Results of a cross between two species of Poly to/// a . . . 736 



171. New combinations resulting from crossing over in Polyto///a 738 



172. Diagram of the sex chromosome of Polyto/zia pascheri plus 



and P. uvella minus 743 



173. Diagram of the sex chromosomes produced by crossing over 



between the chromosomes of Polyto//ia pascheri plus and 



P. uvella minus 744 



174. Diagram of pregamic divisions and pronuclear exchange in 



ciliates 751 



175. The four clones of Para///eciu/// produced from the two ex- 



conjugants of a pair, in the experiments of Sonneborn and 



of Jennings 753 



176. Change of size resulting from conjugation of individuals of 



large and small races of Para///eciu//i caudatu//? .... 762 



177. Changes in mean size of the descendants of the two members 



of an unequal pair of Para///eciu//i caudatu/// .... 763 



178. Different ultimate mean sizes reached by descendants of dif- 



ferent pairs from crosses of the same two races of Para- 



nieciu/// caudatu/// 764 



179. Regeneration in. Uro/2ych/a tra/isfuga 776 



180. Regeneration in Ur onychia uncinata 783 



181. Reincorporation in Difflugia pyrifor/zjis 794 



182. Divisional and physiological reorganization in Uronychia . 797 



183. Diagram showing delayed regeneration in Para/neciu/// cau- 



datum 799 



184. Successive stages in the regulation of the mouth in a piece 



of Spirosto/nu//j with the mouth at the anterior end . . . 803 



185. A relatively mature colony of Zootha/nnium alternans . . 806 



