10 GENERAL EMBRYOLOGY 



Among most of these colonial Flagellates, and indeed in 

 many other Protozoan groups, at irregular intervals these 

 gonidial cells lose the property of directly giving rise to new 

 colonies, and become very highly differentiated in structure 

 and in behavior. These highly modified gonidia are termed 

 gametogonidia, the ordinary gonidia being then given the name 



FIG. 9. Colony of the Flagellate, Pleodorina illinoisensis. Lateral view. 

 After Kofoid. A, anterior; g, gonidial (reproductive) cells, v, vegetative 

 cells. 



of parthenogonidia. The gametogonidia form specialized cells 

 termed gametes which must meet and fuse in pairs, i.e., con- 

 jugate, before reproduction may proceed. In the simplest cases 

 these two gametes are nearly or quite alike. In most cases, 

 however, gametes of two very unlike forms are produced by 

 different gametogonidia. These must conjugate in pairs of 

 unlikes before reproduction is possible. A series of forms 

 illustrating the progressive differentiation of the gametes, or 

 germ cells, is described in Chapter V. For the present we may 

 merely notice that in such a colonial form as Volvox, under 

 certain conditions, the parthenogonidia cease reproducing; 

 certain of them (odgonidia or ovaries) enlarge, and each differ- 

 entiates a large non-motile cell, the ovum, or odsphere, or 

 macrogamete, while others (spermagonidia, or spermaries) after 

 enlarging similarly, divide repeatedly, each forming a large 

 number, often as many as one hundred and twenty-eight, 



