ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 57 



before this is effected one cell at the upper pole becomes constricted 

 off, and gives rise to the " calotte-shaped " cells. In all the species 

 examined there was found an investing membrane ; at the periphery 

 of the homogeneous cell-aggregate there is differentiated a layer of 

 flattened cells, from which in D. tereticoUe arises a structureless 

 cuticle with eight plates beset with chitinous setae ; in all the other 

 forms examined there is a ciliated membrane, in which it is not 

 possible to demonstrate the presence of separate cells. The solid 

 endoblast lying within the flattened ectoblastic cells is at first made 

 up of similar cells, but in the course of development some of them 

 become flattened out and form a kind of epithelial lining to the ecto- 

 blast ; others, at the cephalic end, become fashioned into an enteron, 

 the lumen of which is formed by the gradual degeneration of the 

 inclosed cells. The great mass of the endoblast remains, however, 

 unaltered, and gives rise to germinal cells. 



Some young forms are provided with vessels in which ciliated 

 infundibula are to be made out at certain points; in Distomum cygnoides 

 and D. tereticoUe these appear to be connected with the enteron. 



The investing membrane is regarded by the author as a formation 

 of the ectoblast which is developed in two successive layers: first 

 there appear cells which may be called ectoblasts of the first order • 

 these are replaced by the ectoblastic cells of the second order — the 

 permanent embryonic ectoderm. It may be as well to remind the 

 reader that Van Beneden has noted the same phenomenon in the 

 Tseniadae. 



Further investigations are required to answer the question as to 

 whether the muscles are truly of epithelial origin, or whether they 

 have a mesenchymatous character ; and the first point to be settled is 

 the relation of the young stages of Trematodes to the Enterocoelia. 



The author does not share the oi^inion of Leuckart as to the 

 mesodermal character of the germinal cells; it is perhaps best to 

 regard them as cleavage-elements which have not been used up, and 

 which, being such, do not require any further fertilization. The 

 germinal cells found in the first set of Bedim may similarly be 

 looked upon as cleavage elements which have remained over from 

 the first generation. 



The resemblances between the developmental histories of the 

 Trematoda and of Malacohdella are perhaps superficial. There is, 

 again, a close resemblance between the Trematodes and the Meso- 

 zoa, and the only difference between the embryos of Bhopolura and 

 of Distomum is that the former remains at a lower grade of 

 development. The existence of the former in a highly nutrient 

 fluid makes the development of any other organs than those of 

 reproduction altogether superfluous : the organs which are developed 

 in the Trematoda are such as are necessary to enable it to find a 

 new and suitable host. The Distomidae and the Orthonectidee are 

 therefore, in all probability, closely allied, though the direct origin 

 of the former group is a more difficult question. 



There are many very striking and important resemblances in 

 the developmental history of the Tseniadae and the Trematoda : 



