82 THE ANATOMY AND DEVELOPMENT 



I have in many instances found this matter forming upon the 

 external coats of young secondary hydatids, which were con- 

 fined, as above stated, in old and degenerating parent sacs. In 

 general this cretaceous matter originates in the internal and 

 germinal membrane of the parent sac ; these two membranes in 

 old hytadids being always thick, gelatinous, and homogeneous, 

 like pure gelatine. This thick gelatinous membrane presents no 

 trace of the two membranes of which it originally consisted ; it 

 is generally about the eighth of an inch in thickness ; and lies 

 in the most dependant part of the cavity, quite loose and de- 

 tached from the external coat. It presents no trace of young 

 vesicles or hydatids, but has upon its internal surface a number of 

 white, opaque, fatty looking spots of all sizes. Similar spots, but 

 of much smaller size, are also to be seen in the substance of the 

 membrane, and when examined by the microscope, present a 

 peculiar cellular network. As these spots become larger, they 

 from being quite smooth, become rough and nodulated, each of 

 the cells being apparently filled with the peculiar fatty substance. 

 As this mass encreases in size, it becomes more cretaceous, and 

 sends out branches in all directions, so as in time to fill the 

 whole cavity of the hydatid, which, as this process is going on, 

 shrinks up very much, so that it meets the fatty matter, and 

 enables the process of filling up to be more speedily completed. 

 Shortly before the cavity is completely filled up, the fatty matter 

 begins to lessen in quantity, being probably absorbed by the 

 cretaceous matter gaining the preponderance. In this way more 

 or less of the whole mass is absorbed, so that ultimately nothing 

 is left but a small quantity of cretaceous matter which becomes 

 very much condensed. 



The middle membrane then appears to play the most import- 

 ant part in the economy of the hydatid ; the external membrane 

 acting only as an organ of defence. 



Of this peculiar form of animal three species have been deter- 

 mined, the characters of which are derived from the structure 

 and appearance of the germinal membrane. In Acephalocystis 

 simplex, the lowest of these forms, the whole structure of the 

 animal is much more homogeneous, transparent, and gelatinous 



