EMBRYOS OF THE BOTHRIAD.E. 329 



velopmental history, that this mantle-like structure is nothing else 

 than the further development of that same peripheral cell layer, with 

 which we are already familar in the embryos of the Taeniadse. 



What we know regarding the embryonic development of the 

 Bothriocephali is due mainly to the investigations of Kolliker 1 and 

 Mecznikoff, 2 which both refer to Bothriocephalus proboscideus (B. 

 salmonis), whose embryos are produced in the mother, and whose 

 other conditions, in spite of the divergent nature of the ovary, agree 

 in all important respects with the embryonic history of the Tasniadse. 



In the Bothriadae also it is only the germ-cell (germinal vesicle of 

 Kolliker) which forms the embryo ; the granular yolk-mass has no 

 more direct share in its formation than had the more albuminoid 

 matter of the Tcenice. A spherical mass of cells results from a com- 

 plete and regular segmentation. This becomes differentiated into a 

 central nucleus and a peripheral layer, as we have seen in some Tcenice. 

 This peripheral layer becomes the above-mentioned embryonic enve- 

 lope, which is destitute of cilia in Bothriocephalus proboscideus, but is 

 nevertheless homologous with the similarly originated ciliated covering 

 of B. latus. During the transformation of the nucleus into the body 

 of the embryo this layer loses (according to Mecznikoff) its early 

 cellular structure, and becomes a simple cuticular membrane enclosing 

 the embryo. 



As to the comparative embryology of this peripheral layer, we can 

 only liken it to the ectoderm. This ectoderm, however, is never 

 retained in later life. Even the species with a ciliated coating are 

 stripped of it as soon as they migrate into their host. Thus we 

 conclude that the tape-worms have no ectoderm a conclusion to 

 which we were formerly led by histological considerations (p. 289), 

 which forced us to deny the existence of any true subcuticula. 



The statement of the older van Beneden, that some Bothriada3 

 liberate their eggs in the Scolex form, is utterly without foundation, as is 

 also the assertion that the embryos are in some cases without booklets. 

 We have indeed come to know of a form in the genus Amphiline in 

 which ten hooks are present, 3 instead of the usual six, or the four which 

 occasionally occur in Bothriadae. It is, however, doubtful whether 

 these forms ought to be referred to the Cestodes (p. 268). Yet it is 

 interesting to note that even among the genuine representatives of the 

 Cestodes embryos are sometimes found which have more than the 

 normal six hooks. Thus Heller mentions instances of embryos of Tccnia, 

 saginata with twelve, sixteen, and even thirty-two hooklets, some of 



1 Mullcr's Archivf. Anat. u. PhysioL, p. 91, 1843. 



2 Bull. Acad. imptr. St. Pttersbourg, t. xiii., p. 290, 1869. 



3 Salensky, Zeitschr. f. wiss. Zool., Bd. xxiv., p. 291, 1871 



