UROGENITAL SYSTEM OF MYXINOIDS 83 



mals. The writer found that there is a correlation between the 

 jSize of the pronephros and the length of the animal in Myxine, 

 but not a sufficiently large number of specimens was available 

 to prove this to be constant. The largest pronephroi (those of 

 Myxine no. 10) are in an animal which has the appearance of 

 being quite old. The skin is darker than usual, very thick, 

 WTinkled and hard, the muscle fibers in the body w^all are large 

 and coarse, the walls of the alimentary canal are very thick, the 

 liver and gall bladder are very large and coarse. In this speci- 

 men the pronephroi are a very dark brown color; in all the other 

 specimens and in all the Bdellostoma examined the pronephros 

 is gray. It is probable that the size of the pronephroi is more 

 correlated with the age than with the length of the animal. 

 The right pronephros of Myxine was almost always larger than 

 the left. 



Price ('04) found that at some stage of embryonic development 

 of Bdellostoma stouti an excretory tubule appears in each somite, 

 from the 11th to 13th to the 79th to 82d. That is to say, in 

 the embryo pronephric tubules are present in the same seg- 

 ments with gills. As the gills shift posteriorly (Dean, '97) these 

 pronephric tubules are pushed along behind them and eventually 

 become crowded together in a ''small, compact body, the pro- 

 nephros of the adult, in the region a little posterior to the thir- 

 tieth segment" (Price, p. 132). Price does not mention anything 

 in regard to the history of the veins connected with the tubules 

 in this movement posteriorly. 



As w^ould be expected, there is a limitless variety in the minute 

 structure of the pronephroi. It is well known that each proneph- 

 ros is composed of a large number of small tufts or lobules of 

 tubules which open separately into the pericardial cavity. Usu- 

 ally there are fewer of these lobules at the anterior than at the 

 posterior end, and one or two lobules may be entirely separated 

 anteriorly from the rest of the pronephros. Figure 12 repre- 

 sents a reconstruction on millimeter paper of the right proneph- 

 ros of Bdellostoma no. 15. This pronephros was cut in trans- 

 verse sections, each 10 microns thick, and it will be used as a 



