96 JESSE LE ROY CONEL 



any place where the latter is not surrounded by the central mass, 

 nor do they occur in all those segments of the duct which are 

 ensheathed by the central mass. There is no regularity in the 

 location of these openings, except that they are always confined 

 to the posterior half of the pronephros. As is shown by figures 

 20, 21 and 22, by means of these openings the lumen of the pro- 

 nephric vein is in actual communication with the pericardial 

 cavity through the central duct and the tubules. Near the open- 

 ings blood corpuscles are numerous in the lumen of the central 

 duct, and they extend far up the tubules toward the openings of 

 the latter into the pericardial cavity. In one specimen blood 

 plasma extends from the central duct two-thirds of the entire 

 length of four or five tubules. The pericardial cavity communi- 

 cates with the peritoneal cavity through the large pericardo-peri- 

 toneal foramen, and the peritoneal ca\dty opens into the cloaca 

 through the genital pore, hence we have in Myxinoids the strange 

 condition of the vascular system being open to the exterior of 

 the body. The writer found no traces of blood in the pericardial 

 cavity. Since there are many hundreds of tubules in each adult 

 pronephros, it is difficult to explain why blood is not poured into 

 the pericardial cavity. Price ('10) found that carmine grains 

 injected into the peritoneal cavity through the genital pore of 

 living animals were present later in abundance in the blood taken 

 from different parts of the body. He presumed that the carmine 

 grains gained admission to the blood stream through the cili- 

 ated pronephric tubules, that is to say any current in the tubules 

 which is caused by cihary action is from the exterior toward the 

 interior. This inference is corroborated by the fact that, in al- 

 most all the tubules examined by the writer, the free ends of the 

 granular processes extending from the epithelial cells of the 

 tubules are directed inward toward the central duct. 



In his description of the dievelopment of the pronephros Price 

 ('04, p. 137) states that the manner in which the central duct 

 becomes shortened as the tubules are crowded together is a point 

 which has not been worked out; bending of the duct will not ac- 

 count for all of it. Kirkaldy ('94, p. 356) thinks the duct breaks 

 down and becomes the central mass. Morphological evidence 



