34 ON PARASITES. 



and a complicated ovary. One part of the ovary forms the germinal 

 vesicle, the other forms the vitelline suhstance. The germigine and 

 vitelligine are both double. The two parts of the vitelligine unite in 

 a common canal, into which open the two divisions of the germigine. 

 At the suitable period the intromittent organ is everted from its 

 sac and introduced into the vagina ; the zoosperms are then deposited 

 in the vagina whence they are conveyed into the copulatory vesicle. 

 Intromission having been accomplished the penis is restored to its sac. 

 Now the germinative vesicles may be seen passing successively towards 

 the matrix. When they arrive at the opening of the vitelligine they 

 receive a covering of vitelline substance which is coincidently injected 

 into the canal of the germigine. Enveloped in vitellus the germina- 

 tive vesicle passes on, and the two elements being combined, the resul- 

 tant ovule is prepared for fecundation. As the ovule passes onward 

 toward the matrix it appears before the orifice of the copulatory vesi- 

 cle, then one or more zoosperms are coincidently ejected from the 

 vesicle and coming into actual contact with the ovule fecundate it, as 

 it continues its course towards the uterus. The uterus gradually fills 

 with fecundated ovules, and as it enlarges by excentric pressure it 

 encroaches upon the parenchyma of the proglottis, so that finally the 

 proglottis becomes little more than an egg sac. The ovulation being 

 complete, the proglottis either separates from its younger neighbours 

 anteriorly or remains in connection with them. In either case when 

 the suitable conditions for development are presented, and they are in 

 the intestines and the outer world, the segmentation of the vitellus and 

 disappearance of the germinative vesicle take place precisely as in 

 other animals. The mulberry appearance in due time appears, and 

 the variously hexagonal cells divided from the segmentation of the 

 vitellus, form the layer beneath the vitelline membrane which is called 

 the blastodermic membrane. The remnants of the vitellus with some 

 serous fluid occupy the interior of this membrane. Now the area 

 germinativa appears, and cells heaping and condensing together in 

 this area gradually assume the shape of the famed six-hooked 

 embryo. The mature proglottides, ^. e. those with developed embryos, 

 singly or several united together, escape into the outer world with the 

 faeces, and unruptured (occasionally ruptured). In the outer world 

 they migrate in various directions, appearing upon grass, in the water, 

 &c. li they have been deposited by birds with their faeces upon the 

 foliage of trees they will be found in such localities. Also, if the 



