298 



THE ANIMAL PARASITES OF MAN 



layers, arise, so that eventually it is difficult to say whether the whole 

 egg is present, and, if not. what the layers that remain really are. 



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 water. Separate cells or a layer of 

 cells always separate from the segmentation cells, as well as from the 

 cells of the developing embryo, 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, but more correctly the 

 embryonal shell or embryophore. In some species it carries long cilia, 

 as in Dibothrioceplmlns latus, by aid of which the young swim about 

 when released from the egg-shell ; as a general rule, however, there 

 are no cilia and this envelope is homogeneous, or is composed of 

 numerous rods and is calcified, as in Tcenia spp. (fig. 197)- The 

 second outer envelope ("yolk envelope") (fig. 207, 3) lies close within 



FIG. 195. Egg of Diplogono- 

 $orus grandis, showing the 

 morula surrounded by yolk cells 

 and granules. 440/1. (After 

 Kurimoto.) 



FIG. 196. Uterine egg of Tcenia saginata, G. 

 Uterine shell with filaments ; the oncosphere with 

 embryonal shell (embryophore) in the centre. 

 500/1. (After Leuckart.) 



the true (ootype) egg-shell, and remains within it when the embryo 

 hatches out, and in many species, as in Tcenia spp., it perishes at the 

 end of the embryonal development with the delicate egg-shell which 

 was formed in the ootype, so that one observes not the entire egg 

 with egg-shell but only the embryo in its embryonal shell, viz., the 

 embryophore (fig. 197, a.). 



The embryo (the ONCOSPHERE) enclosed within the embryonal 

 shell (embryophore) is of spheroidal or ovoid form (fig. 197, 6.), and is 

 distinguished by the possession of three pairs of spines, a few terminal 

 (flame) cells of the excretory system, and muscles to move the spines. 



No FURTHER DEVELOPMENT of the oncosphere takes place, either 

 in the parent organism or in the open ; in fact, in all cases in which 

 the oncospheres are already formed within the proglottids they do 

 not become free, but remain in their shell ; it is only when the 

 oncospheres are provided with a ciliated embryophore that they leave 

 the egg-shell, and they even cast this ciliated envelope after having 



