250 



PLATYHELMINTHES. 



segments the course of development passed through hy the sexual 

 organs and products in their origin and gradual progress towards 

 maturity. An examination of the segments between that with the 

 first trace of the generative organs and the first proglottis with fully 

 developed organs will give us an idea of the stages of structure 

 through which each segment has to pass. The tape-worms are ovi- 

 parous ; either the embryo develops within the egg-shell in the body 

 of the mother, or the development takes place outside the proglottis, 

 for example, in water (Bothriocephal'm). 



FIG. 202 Ripe proglottides ready to sepa- 

 rate, a, of Taenia soliiim; b, of Taenia 

 saginata. We water - vascular (excretory) 

 canal (from Glaus). 



FIG. 203. Egg with embryo : a, of Taenia 

 solium ; b, of Microtaenia; c, larva of 

 Bothmocepliahis latiis (after R. Leuckart). 



The eggs are round or oval and of small size (Fig. 203) : they 

 consist of the minute ovum embedded in yolk-cells and surrounded 

 by a membrane, which is thin when the development takes place in 

 the uterus, and thick and provided with an operculum when it occurs 

 only after oviposition (Bothriocepholidae, etc.). The early develop- 

 ment appears to be closely similar to that of Trematodes; the 

 segmentation is complete, and is followed by the epibolic formation 

 of two membranes, of which the outer, lining the inner surface of 

 the shell in the BothriocepTialidae (Fig. 204), is called the shell or 

 enveloping membrane, the inner being the so-called ectoderm or outer 

 layer. These two membranes surround the rest of the embryo, 

 which we shall call the inner mass. The enveloping membrane is 



