REPRODUCTION 241 



nuclei which, according to observations of several different authori- 

 ties, later fuse into one (p. 546). 



In all of these cases the specialized structures accompanying 

 spore formation are formed only at one period in the life cycle and 

 a period which comes at the end of long-continued metabolic activ- 

 ity. They represent therefore, a differentiated protoplasm which is 

 not evident in the protoplasmic make up of the progeny. What 

 is true of these visible differentiations is also probably true of 

 analogous differentiations which are not visible, and we have reason 

 to believe that the products of unequal division and of multiple 

 division are not encumbered by protoplasmic conditions which 

 hamper vitality— in other words, that they have undergone reorgan- 

 ization. Such young forms have again the potential of vitality of 

 the genotype and are able to go through the series of differentia- 

 tions which are characteristic of the life of the genotype. 



IV. DEVELOPMENT. 



In Metazoa, development starts with the fertilized egg and con- 

 sists in the progressive formation of organs and organ systems by 

 differentiations, and grouping of differentiated cells. A strict com- 

 parison of Protozoa with Metazoa in development would involve the 

 history of a fertilized cell through all phases of asexual reproduction 

 (comparable with somatic cell division) to the gamont stage. Only 

 by a fanciful interpretation, however, can the entire progeny of a 

 single fertilized cell of Protozoa be regarded as an individual similar 

 to a metazoon, although there are similar phases of vitality which 

 may be indicated in common by the terms youth, maturity and age 

 (see Chapter VII). The protozoan "individual," however, is a single 

 cell and as usually seen is in the agamont stage. In the majority 

 of Protozoa little or no development is necessary, the daughter cells 

 being almost perfect individuals when formed and similar enough 

 to the parent to be mistaken for nothing else. Here the only pro- 

 cesses that can be regarded as development are those which have 

 to do with the formation of shell structures, as in Coleps hirtus, 

 etc., and the new development of anterior parts of posterior daughter 

 cells and posterior parts of anterior cells. 



It is quite different, however, with the products of multiple bud- 

 ding or of multiple division. Here the young forms are unlike the 

 parent, and during growth undergo changes which may properly 

 fall under the heading of development. In some cases, for example 

 in Foraminifera, Mycetozoa, and Sporozoa, the small fragments 

 produced by a parent may or may not require fertilization in order to 

 develop. The zygote of Polystomellina crispa or of Trichosphaerium 

 sieboldi, formed by the fusion of flagellated gametes (flagellispores) 

 develops into the asexual generation by protoplasmic growth and 

 16 



