REPRODUCTION AND DEVELOPMENT 



561 



Figure 400. A turkey gobbler without a father. 

 This photograph was taken when the bird was 32 

 weeks old and weighed about 15 pounds. (Photo 

 courtesy of M.W. Olsen, U.S. Department of Agri- 

 culture, Beltsville, Md.) 



Paedogenesis 



The larvae of the gall fly (Miastor) pro- 

 duce eggs that undergo parthenogenetic de- 

 velopment to produce other larvae. Such 

 parthenogenesis among larvae is called 

 paedogenesis. 



Neoteny 



The larvae of the tiger salamander under 

 certain conditions will become sexually ma- 

 ture, mate, and produce fertile eggs; this 

 phenomenon is known as neoteny. 



Evolution of sex 



In one order of flagellated protozoans, 

 the Phytomonadina, which are considered 



by many to be plants, are a group of types 

 which can be arranged in such a way as to 

 illustrate different grades of organization 

 from a loose association of cells to one ap- 

 proaching a metazoan, and also to illus- 

 trate the evolution of sex. 



Spondylomorum 



Spondylomorum (Fig. 401) is colonial 

 in habit with 16 cells in each colony. These 

 cells are practically independent; each repro- 

 duces by fission to form a colony like the 

 parent colony. No gametes are known. 



Chlamydomonas 



Chlamydomonas (Fig. 401) is a unicel- 

 lular type that reproduces asexually by sim- 

 ple cell division (fission) into 2, 4, or 8 

 daughter cells and also produces gametes 

 of one size, which fuse together in pairs; 

 the resulting zygotes undergo fission, result- 

 ing in a number of unicellular individuals. 

 In this organism and similar forms, we ap- 

 parently observe the origin of sex cells and 

 of sex. 



Pandorina 



Pandorina (Fig. 401) is a colonial form 

 of 4 to 32 cells embedded in a gelatinous 

 matrix, each independent of the others. New 

 colonies are asexually produced by division 

 of each cell in the mother colony into a new 

 daughter colony. In sexual reproduction, 

 gametes of unequal size are formed; the fe- 

 male gamete is the larger. Male and female 

 gametes fuse to form zygotes; and since 

 sexual reproduction usually consists of the 

 fusion of a larger female gamete with a 

 smaller male gamete, the process in Pan- 

 dorina seems to furnish an early stage in 

 the evolution of sex. 



Eudorina 



Eudorma (Fig. 401) is a colonial type 

 with 32 cells. Each cell may reproduce a 

 colony asexually by fission. At times the 

 cells of certain colonies become large; this 



