56 ANIMAL PAEASITES. 



the young embryo is materially facilitated and its further deve- 

 lopment favoured. 1 



As soon as it has in this way rather rapidly enlarged to 

 a certain size, and arrived at a state of repose, the round 

 vesicle thus formed, when it does not project freely into a serous 

 cavity of the body and has not fallen into one, surrounds itself 

 with a peculiar envelope, which protects it from external pres- 

 sure, and assists it in procuring its necessary repose. These 

 enveloping cysts, as is proved even by the relations of the finer 

 structure of the cyst, constitute an absolute new formation, 

 which commences around the germ of the vesicular worm from 

 the same masses of exudation which are deposited around the 

 youngest brood during its migrations ; this new formation acquires 

 a structure analogous to that of the subjacent tissues, in this case, 

 therefore, analogous to that of the serous membranes, and con- 

 stantly increases in size with the growth of the young Cestode 

 vesicle. 



These enveloping cysts always exhibit the same chemical re- 

 actions as other serous membranes ; they dissolve in caustic 

 potash, with the exception of a very small rolled up membranous 

 film, which, however, is not chitine, as may be ascertained by the 

 employment of Von Bibra's method for the detection of chitine. 



As regards the histological structure, we very clearly find all 

 those elements which Luschka describes and figures, (' Ueber 

 Structura der serosen Haute/ Taf. I, fig. 4), which make their 

 appearance with particular distinctness after treatment with 

 cold caustic potash. Besides the elastic fibres and cellular 

 fibres, we find those formerly called by Luschka " serous fibres/' 

 but subsequently denominated by him " plastic fibres of the 

 areolar tissue." It is indeed in these enveloping cysts that we 

 may trace the various steps in the development of the cells of 

 ligamentous tissue into fibres, and the formation of the so widely 

 diffused plastic fibres by the direct splitting of plastic striae. 

 Thus we see 1. Cells of ligamentous tissue which pass from an 



1 It may also be possible that besides the structure, a difference of function may also 

 occur in the enveloping cysts in cold and warm-blooded animals. It is a universally 

 admitted observation that in warm-blooded animals the secretion furnished by the serous 

 membranes is reabsorbed by them with great difficulty. Should this take place with 

 greater ease in cold-blooded animals we should have in this a further explanation of the 

 variations of the caudal vesicle in true cystic worms. 



