WOLFFIAN DUCT AND BODY IN THE CHICK. 23 



chick in which thirty-six protovertebrse could be counted, but 

 possibly there were more. 



The glomerulus has grown immensely (figs. 19_, 20, 21), and 

 •has now acquired the peculiar histological features which 

 characterise it at the time of its greatest development, and which 

 have already been described in a former paper. 



Anteriorly the bay has widened out considerably (fig. 19), 

 and the glomerulus {e. gl.) projects directly into the body 

 cavity. Posteriorly the bay remains deep (figs. 20, 21), and the 

 glomerulus almost completely fills it and projects beyond it into 

 the body cavity. In sections behind fig. 21 there was seen a 

 fairly well-developed internal glomerulus. 



The edges of the bay are gathering round the glomerulus pre- 

 paratory to fusing with it, and so closing up the peritoneal 

 funnel and dividing the glomerulus completely into two parts, 

 the internal vascular tissues of which, however, are continuous. 



In this stage the epithelial covering of the external glomerulus 

 [e. gl.) was distinctly, as in the previous stage, continued behind 

 directly into that covering the posterior internal glomerulus. 



When, however, the peritoneal funnel closes by the comple- 

 tion of the process commenciog in figs. 20 and 21, this epithelial 

 continuity is lost, and we have the final stage of the glomerulus, 

 the last which I have observed, in which the separation above 

 described is complete, so that in this stage, which is that of the 

 greatest development of the external glomerulus, and corre- 

 sponds with the commencing formation of the head-kidney, the 

 glomerulus belonging to one tubule is divided into three parts. 



(1) An anterior^ part projecting into the body cavity. This 

 corresponds to a further development of fig. 19. 



(2) A middle part, continuous with (1), also projecting 

 freely into the body cavity, but also connected by vascular 

 structures with an internal glomerulus. This part is figured in 

 fig. 26, and corresponds to a further development of the part 

 from which fig. 20 and 21 were taken. 



(3) A posterior part, in which there is no external glomerulus, 

 but merely an internal one belonging to a true Malpighian 

 body of the mesonephros, which I have not thought it neces- 

 sary to figure in this or the previous stage. It is a further 

 development of fig. 18. This stage, which may be observed 

 about the middle of the fourth day of incubation, brings to a 

 close my observations on this extraordinary structure. It appears 

 that in the chick the stage just described is that of the greatest 

 development of the external glomerulus. In the duck, however, 



I Fig. E, PI. 11, in the paper on the ' Head-Kidney of the Chick : 

 Studies from the Morphological Laboratory in the University of Cambridge,' 

 Part 1, 1880, and ' Quart. Joarn. Mic. Sci.,' vol. xix. 



