276 Causes and Course of Organic Evolution 



spheric. Tlie latter seems to correspond to tlie trophic stage 

 in Trichosphcerium while the former gives rise to broods of 

 swarm-cells which escape, conjugate with each other, and 

 develop a microspheric generation. The last may produce 

 in turn either a megalospheric or microspheric crop of new 

 forms. 



In the Heliozoa conjugation occurs after encystment, and 

 in such cases as Aciinophrys the resemblance to what happens 

 amongst desmids is considerable. Exact details are lacking 

 in most genera as to the minor changes. 



In the Radiolaria the formation of ciliated isospores and 

 heterospores, as also of cihated microspores and megaspores, 

 has already been noted, but, though conjugation may occur 

 between some of these, such has not yet been observed. 



From the standpoint of sexuality the Sporozoa and Hsemo- 

 flagellata are semi-parasitic or parasitic groups, each of wliich 

 has either had a polyphyletic ancestry as proposed })y Wood- 

 ward (108, 1: 246) for the latter, or if each group is mono- 

 phyletic it has undergone varied and advancing modification 

 in the sex-cells, that have given rise to an instructive series. 

 It should however be borne in mind that our knowledge of 

 both groups is still very limited and imperfect, in spite of the 

 great advances made in the past quarter century. 



The group Sporozoa presents the most varied advance, 

 from simple conjugation of like gametes, to the union of sperm 

 and egg constituents. In the subdivision of the Gregarinida, 

 the genus Monocystis has been minutely observed. Here two 

 adult individuals surround themselves by a common cyst. 

 The nucleus of each becomes resolved into chromatin or chro- 

 midial portions, that seem at least in part to disintegrate. 

 The remainder reforms as a new nucleus, that then divides 

 into numerous small nuclei. These attract the surrounding 

 protoplasm of each individual, so that it becomes divided 

 up into many oval or irregular bodies, which surround a cen- 

 tral residual mass. These bodies become isogametes, and 

 through breaking down of the parent cyst- wall they mingle 

 and conjugate. Whether two individuals, derived from the 



