The development uf tlie urinogenitiil oif^aus u( the lauiprey. 31 



or into the body-cavity if it arises from a cell in the very orifice of 

 the uephrostome. This fiagelluui is rather rigid uear its base but 

 becomes more flexuous towards its tip as shown on the figures. "Er- 

 satzzellen" also occur in the walls of the funnels (ec) quite as frequently 

 as in the walls of the tubules, and indicate that these cells, too, are 

 destined to be replaced. 



When we examine the collecting and pronephric ducts of this 

 same Ammocœtes 9,5 cm long, we find the latter extending back with- 

 out interruption through the mesonephros to the cloaca. Its diameter 

 is very small, to be sure, just behind the pronephros as c(*mpared 

 with its calibre in the region of the mesonephros, but still its lumen is 

 large enough to carry otf the secretion which is passed over to it by the 

 collecting duct. Histologically it closely resembles through- 

 out its length the pronephric tubules. It stains very faintly, 

 its cells consist of granular protoplasm with striated free surface, and 

 the rounded nuclei and "Ersatzzellen" occur throughout its length as 

 well as in the collecting duct. It is surrounded more or less com- 

 pletely throughout its length by a capillary net- work. Sometimes it 

 seems to lie within a venous sinus, so that in this respect, too, it 

 resembles the pronephros. 



The Ammocœtes 17 cm long shows the concluding stages in the 

 atrophy of the pronephros. The organ is still large, at least the pro- 

 nephric lobes have preserved their outline , but sections demonstrate 

 that their internal structure has undergone great changes. The cardinal 

 veins have become very voluminous and are packed full of blood- 

 corpuscles. Crossing the lumen of the vessel are seen strands of 

 tissue containing pigment. These are the remains of the vascular 

 walls which covered the tubules when they had come to lie within 

 the vein. In one of the pronephroi there are no traces of tubules or 

 funnels. Only a few small accumulations of deeply staining cells 

 among the paler blood-corpuscles, may perhaps represent the tubule- 

 cells in process of dissolution. The glomus is scarcely recognizable 

 as a somewhat homogeneous plasmatic mass containing a few nuclei. 

 The other pronephros is not so completely atrophied. One funnel and 

 a piece of its tubule are still distinctly recognizable, though their 

 component cells are much distorted. The glomus, too, still retains 

 much of the structure seen in younger Ammocœtes. On both sides, 

 however, the pronephric duct has been severed from the pronephros 

 and ends blindly a short distance in front of the mesonephros as a 

 much attenuated tube. 



