150 DEVELOPMENT OF THE TADPOLE. 



pronephros. It consists of a convoluted tube, terminating in a 

 glandular hollow bulb. Further sections of this tube show its 

 structure to be more complicated than that given in the diagram, 

 as the lower sections proved the existence of several of these 

 bulbs, some round, whilst others were oval, and one or two pos- 

 sessed the appearance of a dumb-l)ell, with a very short, wide 

 handle between the knobs. The tube and its openings is entirely 

 surrounded by a membranous sheath, consisting of long, elliptical 

 cells. The convoluted tube or duct is itself composed of a large 

 number of flat, oval cells placed closely side by side, having much 

 the appearance of an immense number of tiny, flat pebbles, 

 arranged side by side, with their flat iaces touching each other and 

 their rounded edges pointing upwards. The internal portion of 

 the cells appear;; considerably darker than the external ; indeed, 

 the appearance is such as would be presented by a very minute 

 layer of epiblastic cells, surrounded by one of hypoblastic ; but 

 whether this is the case I have not been able to determine, as 

 there appears no definite line of demarcation between the dark 

 interior and lighter exterior of the tube-wall, such as should be 

 due to the approximation of these two difterent classes of cells. 

 This primitive kidney is the same structure, more fully developed, 

 which was shown in Fig 5, PI. ATI., and there called a pro-renal 

 duct. 



Gotte states that the development of this duct commences 

 from a layer of the peritoneal epithelium, its front end being, as 

 here shown, immediately behind the branchial arches, and also 

 that it has free communication, by means of its openings, into the 

 body-cavity. DoubUess, this communication really exists, but at 

 the stage to which we have arrived it can only be through the 

 delicate membrane, if a single layer of cells is deserving this name, 

 which surrounds the whole organ, and separates it from the body- 

 cavity. Indeed, in a section four days older than the one drawn, 

 two of the openings are seen to lie directly against the mem- 

 brane, but the section does not enable me to make out whether 

 they are actually under it or o[)en up into it. These openings 

 have all the appearance of the end of a thickened tube, looking 

 like a thick, dark ring surrounding an opening of about the same 

 diameter as the thickness of the tube-walls. 



