INVERTEBRATA. 



371 



system degenerates. This is the condition of the ripe 

 proglottids at the posterior end of the worm, and these 

 become detached singly or in groups and pass out of the 

 host with the faeces. 



7. Development of Hexacanth Embryo. The segmenta- 

 tion and early development of the ovum take place in the 

 uterus before the detachment of the proglottids. The 

 embryo is formed from only a portion of the total number 

 of segments. The first segmentation forms two blasto- 

 meres, of which one is filled with refringent granules and 

 takes no further part in the formation of the embryo. 

 The other continues to segment and gives rise to three 

 megameres and numerous micromeres. 



The megameres grow larger and form what is called a 

 yolk-envelope. The micromeres form a spherical mass, 

 and two of them at one 

 end flatten out and 

 divide to form a super- 

 ficial envelope, which 

 gives rise to a chitinoid 

 coat. The remaining 

 central mass forms the 

 embryo proper, on the 

 exterior of which are 

 developed three pairs 

 of pointed spines com- 

 monly called hooks. 

 This hexacanth embryo 

 consists of a solid mass 

 of embryonic cells. 

 This is the stage of 

 development reached 

 in the eggs in the ripe 

 proglottid at the time it becomes detached and expelled 

 from the host. The expelled proglottis either decays and 

 sets free the eggs, or it may be swallowed by a pig in its 

 entire condition. In either case the egg envelopes are 

 digested in the stomach of the pig and the hexacanth 

 embryos set free. 



B 



Fig. 192. DEVELOPMENT OF Taenia solium. 

 A, six-hooked embryo ; S, cysticercus ; C 

 ditto with head everted. 



