CESTODES I GENERATIVE ORGANS 2OI 



cesses may occur : (i) Self-impregnation, auto-fecundation (without 

 " immissio cirri "). (2) Self-copulation, auto-copulation (with immissio 

 cirri). (3) Alternative or mutual copulation between proglottides 

 of the same or different chains (of the same species), and (4) 

 mutual or interchangeable copulation in the same proglottis in the 

 species with double genital pores. These various modes have 

 actually been observed. 



In -those species which lack the vagina (Acole'ince) it appears that 

 the cirri, which are always furnished with hooks, are driven into 

 the tissues and thus come into contact with the receptaculum seminis. 



The eggs of all cestodes are provided with shells, but the 

 shells, like their contents, vary. In genera that possess an uterine 

 orifice the mature eggs frequently do not differ from those of the 



FIG. 129. Egg of Diplo- FIG. 130. Uterine egg of Tcenia saginata, G. 



gonoporus grandis. 440/1. Shell with filaments; the oncosphere with 



(After Kurimoto.) embryonal shell in the centre. 500/1. (After 



Leuckart.) 



Fasciolidse ; they have a brown or yellow shell of oval form pro- 

 vided with an operculum, and contain a number of yolk cells in 

 addition to the fecundated germinal cell (fig. 129). In other cases 

 the lid is absent and the egg-shell is very thin. In regard to their 

 eggs, these forms approach those cestodes in which the secretion 

 of the vitellogene gland is a light albumin-like substance that con- 

 tains only a few granules, and the egg-shell is very delicate and 

 without operculum. 



The 'embryonal development in most species takes place during the 

 stay of the eggs in the uterus ; in other species it takes place after 

 the eggs have been deposited and are in the water. Always 

 separate cells or a layer of cells separate from the segmentation 

 cells, as well as from the embryonal cells, and form one or more 

 envelopes round the embryo ; usually two such envelopes are formed, 

 the inner one of which stands in intimate relationship with 

 the embryo itself and is often erroneously termed the egg-shell. 



