250 SUMMARY OP CURRENT RESEARCHES RELATma TO 



he carefully describes the very peculiar mode of connection of the 

 tail with the body; the tail is inserted into a deep pit which is 

 placed at the hinder pole of the trunk, but this tail is not attached by 

 the whole of its anterior surface, but only by two lateral thin fibrous 

 cords ; there is a space between the hinder wall of the pit and the 

 anterior part of the tail. The sporocysts have a cuticular dermal 

 layer, which is, however, really cellular and not cuticular in structure ; 

 the fine diagonal muscular bands, which are very regularly arranged 

 in young, are completely obliterated in older sporocysts. In de- 

 scribing the histological development of the Cercarise the author 

 applies the term of primitive parenchymatous or meristem-cells to 

 those which first appear on the cleavage of the germinal cells. The 

 origin of the muscles from the dermal layer is spoken to by the fact 

 that the early formed rhombic scales have a tendency to form local, 

 regular protoplasmic thickenings. After describing in detail the 

 development of the various organs of the Cercaria, the author sums 

 up the first stages in histological differentiation thus ; the germ-cell 

 by irregular cleavage gives rise to a number of meristem-cells, whence 

 all further differentiations arise; the peripheral cells by gradual 

 metamorphosis and fusion become the cuticle-like dermal layer. In 

 the centre a solid mass of genital cells is formed, from which the 

 ovary, testis, and efferent ducts are later developed. At the anterior 

 pole of the body the meristem-cells are regularly grouped to form the 

 primitively solid rudiment of the fore-gut, the lumen of which arises 

 by the absorption of the axial cells. The enteric limbs arise 

 secondarily from the fore-gut. The space between the integument 

 and the genital mass is filled by meristem-cells, from which the 

 excretory organs, nervous system, &c., arise. A comparison of the 

 first phenomena in the development of the Cercaria with those of the 

 embryo leaves no doubt of their homology. From this it is clear that 

 we must not, with v. Siebold, regard the embryo as an ovarian in- 

 vestment capable of becoming an animal, but as a Distomum which 

 has remained at an early stage of development ; the " germ-cells " of 

 the embryo are the "genital cells" of Cercarise; and the mode of 

 development is not strict asexual reproduction, but true partheno- 

 genesis. The whole cycle of development in the Trematoda has an 

 interesting analogy in Cecidomyia among insects. 



The history of Cercaria ornata, which is found in Planorhis 

 corneus, is next considered; it is very near to C. armata, differing 

 chiefly in the size of the suckers, which instead of being subequal 

 are in the proportion of five (oral) to three (ventral) ; the cyst- 

 glands are more sharply defined, the muscular limbs of the central 

 vascular system are nearer in size to the true unpaired vesicle, and 

 the parenchymatous elements are more delicate in structure. 



Cercaria echinata is found in Lymnseus stagnalis, and differs from 

 most in that the young are found in May and June, instead of in the 

 winter months ; by a little pressure on the living animal the nervous 

 system, testes, and ciliated infundibula can be detected ; the dermal 

 layer and subjacent muscle are only feebly developed, as is also the 

 pharynx ; the gut has ho lumen ; the cell-layer around the nervous 



