THE CYSTIC ENTOZOA. 477 



of obliteration has commenced, and in such can easily be de- 

 monstrated 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 inter- 

 venes. 



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 

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

 germinal cells. It may, therefore, be considered as that por- 

 tion of the middle or germinal membrane which has been 

 carried inwards by the rise of the germinal cells in the sub- 

 stance 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 hydatid. At first this vesicle is colourless, but as it 

 increases slightly in size, it becomes opaque, and also carries 

 the internal membrane inwards before it, which in time, as 

 the young hydatid 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 



