MORE COMPLICATED LIFE CYCLES 



115 



Even more marked is the change in trichospherium where the 

 chemical composition of the skeleton parts changes with advancing 

 age. The young forms resulting from conjugation grow into an 

 adult characterized by a gelatinous membrane and radial spicules 

 of magnesium carbonate. This adult reproduces by the formation 

 of pseudopodiospores, which grow into organisms similar to the 

 parent, or after advanced age (presumably) to a second adult type 

 characterized by a firm membrane and entire absence of radial 

 spicules. _ This second type, as in the foraminifera, finally gives rise 

 to flagellispores, the progeny from different parents uniting and thus 



Fig. 42 



Megalospheric (.4) and microspheric (S) shells of Biloculina depressa, Lam. 

 Schlumberger.) Dimorphism is shown by the central chamber c. 



(After 



completing the cycle. Such secondary types are morphological 

 evidences of changed metabolic conditions characteristic of the second 

 period of vitality. The possibilities of similar alternations in the life 

 history of parasitic and pathogenic forms have hardly yet been realized. 

 2. Nuclear Changes at Maturity. — " Chromidia." — While changes in the 

 body form are often characteristic of the second period of vitality, there 

 are great numbers of protozoa in which the external structure gives 

 no clue to the state of affairs within. The nucleus, however, undergoes 

 changes at this period which are not only more widespread throughout 

 the phylum, but are of far more theoretical and practical importance. 

 These changes have to do with the formation of so-called "chromidia" 

 and with the maturation phenomena of the cell (Fig. 43). 



