36 ON PARASITES. 



fixed itself in a suitable locality, it enters the third stage of its 

 development — the formation of the resting scolex. 



THE RESTING SCOLEX. 



This stage consists in the formation of cestoid heads from the em- 

 bryonal vesicle, in a state of rest. The heads, or scolices, may be 

 formed in the interior of the vesicle or upon its outer surface. The 

 embryonal vesicle acting as a foreign body excites an abnormal vascu- 

 lar activity round it, and plastic matter being thrown out, it finally 

 becomes encysted. The cyst is the result of the ordinary processes 

 that are adopted in the organism to repair injury. If the embryo is 

 in a serous cavity it may not have the cyst formed around it. 



The embryo begins to dilate by the absorption of liquid nourish- 

 ment. 



It is now requisite to follow separately, the three modes of develop- 

 ment of scolices, from cystic worms. 



1st. Cysticercus. A single scolex is formed on the anterior part of the 

 globular vesicle. At its anterior extremity, where the six booklets 

 are situated, a depression is formed in the enlarged embryonal vesicle' 

 This depression deepens, and the inverted anterior portion of the 

 limitary membrane looks toward the inner surface of the posterior 

 part of the vesicle. In the bottom of the depression the scolex ia 

 formed inverted. That part of the enlarged embryonal vesicle 

 which is not implicated in the metamorphic changes connected with 

 the head, becomes the well known caudal vesicle. There is a thick- 

 ening and condensation of cells at the bottom of the cephalic pit. 

 These gradually assume the form of cestoid heads, always inverted. 

 The sucking discs, usually four in number, and the proboscis with its 

 crown of hooks. If examined, the head may be seen as it were sit- 

 ting at the bottom of the pit in which it is formed, and may be ever- 

 ted by pressure when it assumes precisely the appearance of the ces- 

 toid head. 



The hooks are usually completed in the fifth or sixth week. The 

 size of the caudal vesicle may be considered on the average about 

 that of a small pea. The scolices or bookless taeniae are formed in 

 the same manner as the others, with the exception of hooks. 



THE C-ENURI. 



The globular enlarged embryonal vesicle, instead of forming a sin- 

 gle scolex anteriorily, forms several, upon various parts of its sur- 



