CXXXVIU THE VOYAGE OF H.M.S. CHALLENGER. 



Eecently Karl Brandt has given a very detailed and fiilly illustrated account of the sporification of 

 the Polycyttaria (L. N. 52, pp. 145-178). I have also had the opportunity during my sojourn in 

 the Canary Islands (1866), in the Mediterranean at Corfu (1877), and Portofino (1880), as weU as 

 in Ceylon (1881), of observing the development of flagellate zoospores from the central capsule 

 of individuals of all four legions : among the Spumellaeia in certain Colloidea, Beloidea, 

 S p h re r i d e a, and Discoidea, among the Acantharia in several Acanthometra and 

 Acanthophracta, among the Nassellaeia in individuals belonging to the Stephoidea, 

 Plectoidea, and Cyrtoidea, and among the Pileodaeia in one CastaneUid. In most zoo- 

 spores I could distinctly observe only a single long flagellum ; sometimes, however, two or even 

 three appeared to be present, but the determination of their number is very difficult. 



216. Alternation of Generations. — A peculiar form of reproduction, which may be 

 designated " alternation of generations," appears to occur generally in the Polycyttaria, 

 but has not yet been observed in the Monocyttaria. All Collozoida, Sphsero- 

 z i d a, and Collosphserida which have hitherto been carefully and completely examined 

 with respect to their develojDment, are distinguished hy the production of two 

 different kinds of swarm-spores, isospores and anisospores. The Isospores (or mono- 

 gonous spores) correspond to the ordinary asexual zoospores of the Monocyttaria (§ 215); 

 they possess a homogeneous, doubly refracting nucleus of uniform constitution and 

 develop asexually, without copulation. The Anisospores (or amphigonous spores), on 

 the otter hand, are sexually differentiated and possess a heterogeneous, singly refracting 

 nucleus of twofold constitution ; they may therefore be distinguished as female 

 macrospores and male microspores. The Macrospores (or gynospores, comparable with 

 the female macrogonidia of many Cryptogams) are larger, less numerous, and possess 

 larger nuclei, which are less easily stained, and have a fine filiform trabecular network. 

 On the other hand the Microspores (or androspores, comparable with the male 

 microgonidia) are much smaller and more numerous, and are distinguished by their 

 smaller nuclei, which have thicker tuberculae and become stained more intensely. The 

 gynospores and androspores are developed in the Collozoida and Sphserozoida 

 in the same individual, but not in the Collosphserida. It is very probable that these two 

 forms of anisospores coj^ulate with each other after theii' exit from the central capsule 

 and thus produce a new cell by the simplest mode of sexual reproduction. But, since 

 the same Polycyttaria, which produce these anisospores, at other times give rise to 

 ordinary or asexual isospores, it is further possible that these two forms of reproduction 

 alternate with each other, and that the Polycyttaria thus pass through a true 

 alternation of generations. This has not yet been observed in the Monocyttaria, and 

 hence these latter seem to bear to the Polycyttaria a relation similar to that in which 

 the sexless solitary Flagellata (Astasiea) stand to the sexual social Flagellata 

 (Volvocinea). In the two analogous cases the sexual difi"erentiation may be regarded 

 as a consequence of the social life in the gelatinous colonies. 



