814 REPOET— 1893. 



The nepbridium of certain beings is an epithelial tube, the lumen of which 

 owes its origin to the disjunction of the cells. This is the case with the vertebrate 

 and other forms. 



In other beings, as for instance the leeches, the organ is not really epithelial, if 

 we give this word its original and etymological signification. It consists of an 

 aggregation of cells with no disjunctive cavity whatever. All the lumma it 

 possesses run through the body of the cells, and thus are wz)!;-acellular ductules, 

 as Professor Lang was the first to remark long ago. In Clepsine, for instance, the 

 whole organ consists of a single row of cells, with three separate canals running 

 through it. These canals end in the upper part of this row of cells by spreading 

 into a bunch of tiny ramifications creeping in the cytoplasm (fig. 1, i.d.). 



In Hirudo the structure is a little more complicated by the fact that a sheath 

 of cells surrounds the central canal. Tlie cytoplasm of these cells contains a 

 number of ramified ductules uniting from cell to cell, and opening at certain places 

 into the central duct. The wall of this latter always shows in a transverse section, 

 only as one single cell ; that is to say, that its lumen is also an intracellular cavity. 



It may be remarked that I do not take the usual diagrams of the nephridium 

 as ordinarily given according to the observations of Bourne, Schultze, and others, 

 but the somewhat different one lately given by Bolsius. I know that Bolsius's 

 diagrams have been criticised by Mr. Bourne, of Madras. Nevertheless, I cannot 

 help considering his views as much more nearly correct than those generally 

 accepted. It is to be hoped that Bolsius will soon give some explanations about 

 his disagreement from Mr. Bourne's descriptions. 



Besides, be these diagrams accepted or not, we may remarlc that, whilst in 

 vertebrates and other forms the nephridium is epithelial, and its lumen inter- 

 cellular, i.e., disjunctive, in the leeches, on the contrary, the organ is not epithelial, 

 and its lumen is not disjunctive, but always intracelhdar. 



It seems to me that differences of this character have a certain importance, 

 at least for the evolutionist, for they imply coiTesponding differences in the pro- 

 cesses of histogenesis. 



The homology of the excretory organs of the different groups of animal forms 

 is, indeed, still uncertain. We do not exactly know which are homogenetic and which 

 simply hoitioplastic, to use two excellent terms proposed by Professor Ray Lankester. 

 Every kind of information about them should be taken into consideration, and it is 

 desirable that a minute cytological survey of the nephridia should be canied out 

 through all groups. 



But whatever may be the residts of these investigations, I do not believe that 

 such differences as those I have been speaking of could ever be regarded as a real 

 objection against the homology, nor even against the homogeny, between the 

 nephridia belonging to various animal groups. 



If we admit, as I believe we must in the hypothesis of evolution, that the non- 

 epithelial state was primary, there is no theoretical difficulty in considering the 

 passage to the epithelial state as the result of a cellular division, which took place 

 at a given moment of the phylogenetic evolution. The vanishing, or better the 

 non-developing, of the internal ductules must then be considered as a second stage 

 of the evolution. 



An interesting confirmation of this opinion lies in the fact that two stages of a 

 similar evolution are found as actual dispositions in other organs, which are, I 

 believe, undoubtedly homogenetic, i.e., the silk glands of insects. I have been 

 engaged in the study of these organs for some time, and found them built on the 

 same plan in four orders, viz., neuroptera, lepidoptera, diptera, and hymenoptera. 

 They consist of two tubes uniting in the head, to form a single canal opening on 

 the inferior lip of the larva. Each of these glandular tubes ordinarily consists of a 

 small number of cells ; it is quite common to find only two cells in a transverse 

 section (fig. 3'). 



But if we examine a similar section of the gland in a peculiar family of 

 hymenoptera, the tenthredinida, we observe a very remarkable difference ; the 

 organ still consists of a tube, the wall of which is composed of flat cells, but, in 

 addition to that, two series of spheroidal cells are attached to the sides. Each 



