30 WILLIAM MORTON WHEELER, 



rarely turned towards the glomus, which secretes this fluid, that one 

 is led to believe that they bear no relation to it. Nor is it at all 

 necessary that they should, since the pericardial cavity functions as 

 a pronephric chamber, or as a large glomerular cavity, closed on all 

 sides excei)t at the nephrostomes. 



Apart from the abundant blood-supply, the minute structure of 

 the diti'erent parts of the pronephros shows unmistakably that the 

 organ is functioning actively. In Fig. 35 I have represented the cross- 

 sections of two of the pronephric tubules from the same slide as 

 Fig. 48 but under a higher magnification. The wall of the tubule is 

 seen to be made of somewhat cuboidal cells which stain very faintly. 

 The cytoplasm is finely granular except next to the lumen, where it 

 is delicately striated after the manner of secreting cells. Occasionally 

 one finds next to the endothelium of the cardinal vein (end) which 

 closely invests the tubule, small triangular cells (ec) differing from the 

 adjacent secreting cells in having still paler cytoplasm and smaller, 

 more deeply staining nuclei. These 1 take to be young cells which 

 will eventually grow out and replace the exhausted secreting cells. 

 They are probably true "Ersatzzellen". Sometimes they contain more 

 than one nucleus, as in the upper portion of Fig. 35 where appearances 

 suggest that these cells may divide by amitosis, especially as 1 have 

 been unable to find mitotic figures even after long search with a high 

 magnification. The lumen of the tubules in these sections is frequently 

 crossed by a plasmatic web, which simulates cilia, but it is in reality 

 only the residue of the secretion from which the water has been 

 withdrawn during the hardening of the tissue ^). 



The structure of the funnels differs in several particulars from 

 that of the tubules just described (Figs. 36a and 36b). In cross- 

 section the funnels, instead of being circular like the tubules, are 

 elhptical (36 b) and bilateral, and this structure extends their full- 

 length to the nephrostome which is distinctly bilabiate as Wilhelm 

 Müller observed. This peculiarity may be seen in Ammocœtes only 

 21 mm long if the pronephros be dissected out and mounted In toto. 

 The projecting funnels look like so many little snakes' heads with 

 gaping mouths. The cells which compose the flattened walls of the 

 funnels are very regularly arranged and deeply columnar with flattened 

 nuclei. Each cell bears a long flagellum which i)rojects into the lumen 



1) Authors have repeatedly mistaken these plasmatic webs for cilia, 

 e. g. Semon in his paper on Ichthyophis (1891). 



