742 SUMMARY OF CURRENT RESEARCHES RELATING TO 



canals ordinarily give off delicate lateral canaliculi, which anastomose 

 with one another and ramify throughout the body. In the JRotifera 

 we find the origin of these ductules ; they open by ciliated infundibula 

 into the coelom. In some Rotifers there are no ductules, but the in- 

 fundibula open directly into the longitudinal canals. In some of the 

 lower Flatworms the ductules have no coelomatic orifice. In the 

 annulated worm the " urinary apparatus is multiple ; typically, there 

 is a pair of organs in each segment," which are placed on either side 

 of the middle line. The coiled tube opens by one or more infundibula 

 into the coelom. In certain types (Gephyrea and embryonic Hirudinea) 

 the two sets of excretory organs are found coexistent. Of late years 

 somewhat similar structures have been observed in adult or embryonic 

 stages of the Vertebrata. The question arises, Are these three 

 sets of organs homologues or only analogues ? After discussing the 

 observations on which we have already reported, the author would 

 appear to incline to the former view. 



Embryonic Development of Taenia.* — M. E. van Beneden enters 

 into a detailed account of T. serrata, and a more summary account 

 of the embryo in T. saginata and T. porosa. In discussing the 

 morphology of the albuminogenous layer which, with the granular 

 layer, forms a complete envelope for the embryo, he affirms its resem- 

 blance to the membrane he previously observed in T. hacillaris. 

 Between T. hacillaris, in which the number of cells is considerable, 

 and T. porosa, in which they are few, we have the two others just 

 mentioned to represent intermediate stages. This envelope is, further, 

 homologous to the ciliated investment of Bothriocpphalus, and both 

 have the same developmental history ; B. prohoscideus has no cilia. 

 The author feels himself justified in supposing that the covering in 

 question is the last vestige of an embryonic membrane which was 

 primitively ciliated, that it lost its cilia, that the cells became reduced 

 in number, and the membrane a mere rudimentary organ. 



The author observes that the ovum of the Tjeniadfe undergoes un- 

 equal segmentation, and he thinks that the albuminogenous cells are 

 the product of a kind of precocious epidermal ecdysis ; the cells tend 

 to envelope, by epiboly, the mass of embryo- cells. 



After discussing the ciliated investment of Bothrioceplialiis as a 

 provisional organ, M. van Beneden points out that the hexacanth- 

 embryo is constituted at first of two layers of cells, a superficial one, 

 in which the chitinous hooks are developed, and a medullary mass, 

 which at first is incompletely covered by the superficial layer. '' Are 

 these two layers homologous with the primitive layers of the other 

 Metazoa ? In the present stage of our knowledge it is not possible to 

 prove that it is so ; but, at least, it is rational to suppose it. Further 

 studies on the organization and development of the hexacanth-embryo 

 are necessary for us to be able to say definitely whether the two layers 

 which I have noticed at the commencement of the formation of the 

 embryo are or are not homologous with the primitive layers of the 

 Gaslrula." 



* Arch, cle Biol., ii. (1881) pp. 183-211 (2 pis.). 



