80 OF THE ANATOMY AND DEVELOPEMEST 



vascular, and forms a cushion, to which the external surface of 

 the hydatid is applied. In this way, a steady supply of the blood, 

 or of debris of the textures of the infested animal is close at hand, 

 from which the hydatid may extract nourishment. This mem- 

 brane is best seen in aged hydatids, or in those in which the 

 process of obliteration has commenced, and in such can easily be 

 demonstrated by dissection. In such aged individuals also it is 

 found to be so intimately attached to the external membrane of 

 the hydatid, as to appear to form one membrane with it ; whereas 

 in younger individuals, a considerable space intervenes. 



The external coat of the hydatid is gelatinous and slightly 

 fibrous in appearance, and presents no structure. 



The middle membrane appears to be of the nature of a ger- 

 minal membrane, is much thinner, and more delicate than the 

 external membrane. In this membrane numerous cells, in 

 various stages of growth, take their rise, and project inwards 

 into the cavity of the hydatid, carrying the next membrane 

 along with them. 



The internal membrane does not appear to be continuous over 

 the whole internal surface ; but observed only where it is re- 

 flected, as has been just stated, over the surface of the germinal 

 cells. It may, therefore, be considered as that portion of the 

 middle or germinal membrane which has been carried inwards 

 by the rise of the germinal cells in the substance of the former 

 membrane. 



A small clear cell, or vesicle, jutting from the internal surface 

 of the second membrane, is the first vestige of the young hy- 

 datid. At first this vesicle is colourless, but as it encreases 

 slightly in size, it becomes opaque, and also carries the internal 

 membrane inwards before it, which in time, as the young hy- 

 datid becomes more pedunculated, before becoming free, almost 

 covers it entirely. Vestiges of this membrane may be seen 

 attached in shreds to the vesicle even after it has attained a 

 considerable size. 



In all the hydatids which have already become independent 

 animals, with their external coat still gelatinous, and are still 

 enclosed within the cyst of the original acephalocyst, it may 

 be observed that one side presents shreds of membrane at- 



