GENERAL CONSIDERATIONS 



^3 



and a micronucleus which has undergone maturation divisions and is 

 haploid as to chromosomes. The final sixteen are associated in pairs, 

 which become surrounded by a cyst membrane within which they fuse 

 (Calkins and Bowling, 1929). For other variations in the phenomena 

 of endomixis, see Diller, 1936; and especially Woodruff, infra, Chap- 

 ter XIII. 



This unusual phenomenon in Dallasia, which was not encountered by 

 Kidder in his studies on Glaucoma, finds its closest parallel in a group 



Figure 12. Glaucoma (Dallasia) jrontata Stokes. History of normal vegetative in- 

 dividual with successive divisions resulting in copulating gametes and encysted zygotes. 

 (After Calkins and Bowling, 1929.) 



of ciliates in each of which the life history is unique. This group includes 

 the ectoparasite Ichthyophthirius multifilih, according to Buschkiel 

 (1911) and Nerescheimer (1908), and O patina ranarum, according to 

 Metcalf (1909) and Nerescheimer (1907). With these forms we have 

 good evidence, together with Dogiel's "transformation of a male pro- 

 nucleus into a spermatozoon" in the Ophryoscolecidae, that the intra- 

 cellular micronuclei forming pronuclei are a reminiscence of a brood 

 of gametes (Dogiel, 1923; see also Turner, injra. Chapter XII). 



