REPRODUCTION 245 



escence. Such young forms have again the potential of vitaUty 

 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 ^letazoa 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 X). 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 Dinoflagellata, 

 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 require fertilization in order to develop. The 

 zygote of Polystoinella crupa or of Trichosphcerium sieboldi, formed 

 by the fusion of flagellated gametes (flagellispores) develops into 

 the asexual generation by protoplasmic growth and nuclear division, 

 but without cell division, development of the former being indicated 

 externally by the formation of a many-chambered shell. Similarly 

 in the Mycetozoa the zygote formed by amoeboid or flagellated 

 gametes develops into a plasmodium by cell fusions and nuclear 

 divisions. 



In the Sporozoa the zygotes, formed by union of similar gametes 

 (isogametes) or of dissimilar gametes (anisogametes) undergo a 

 variable number of metagamic divisions, three in the majority of 

 Gregarinida and two or more in the Coccidiomorpha. The end- 

 result of such metagamic divisions is the formation of two or more 

 similar sporozoites which are entirely different from the adult indi- 

 viduals and undergo a more or less complex development. When 

 they are introduced into a new host the sporozoites are liberated 



