UROGENITAL SYSTEM OF MYXINOIDS 99 



side diameter 0.1620 mm. One tubule in Myxine no. 10 is very- 

 large, the outside diameter being 0.675 by 1.1475 mm. before it 

 breaks up into smaller branches. The smaller tubules have 

 practically no connective tissue between the walls and the peri- 

 cardial sheath, the medium size tubules a httle, while the large 

 ones usually have a comparatively large quantity of connective 

 tissue around them. The smaller tubules have the appearance 

 of being recent branches from the larger ones. 



Kirkaldy ('94) states that, in a Myxine with large eggs, the 

 tubules at the posterior end of the pronephros are entirely differ- 

 ent from those of a Myxine without eggs in that the former are 

 considerably degenerated. The wTiter is of the opinion that 

 this degeneration of the tubules is not so much a matter of be- 

 ing with or without eggs as one of age. Kirkaldy does not state 

 whether the animal without eggs was young or old. In all the 

 pronephroi of Myxine examined by the writer the inner ends of 

 ahnost all the tubules have disintegrated, only occasionally is one 

 found whose walls remain intact to its opening at the inner end. 

 Only in the oldest specimen (Myxine no. 10), which does not 

 have any eggs, are there tubules which have the appearance of 

 degeneration. Some of the largest tubules alone are degenerating, 

 and answer to the description given by Kirkaldy. The nuclei 

 are enlarged, here and there, in the walls, while some of them 

 are attenuated and almost fibrous, and connective tissue is re- 

 placing the palisade cells. These degenerating tubules are not 

 numerous and are not hmited to the posterior end of the pro- 

 nephros. They are surrounded by many smaller tubules which 

 give no evidence whatever of degeneration, but on the other 

 hand look like recently formed branches. The cells of the lat- 

 ter are more uniform in size and shape and stain more deeply 

 than those of the large tubules. 



One of the most striking differences between the pronephroi of 

 Bdellestoma and Myxine is that in the latter the disintegrating 

 process of the interior has proceeded further than in the former. 

 In Bdellostoma the inner ends of the tubules are always entire, 

 while in Myxine they are seldom so. In places only dense lines 

 of nuclei indicate where the inner ends of tubules have been, and 



