ON THE HEAD KIDNEY OF MYXINE. 355 



aggregation of the protoplasmic granules, which seem to be 

 cast off from the cells, along with the mucus apparently 

 excreted by them. Externally a well-defined basement mem- 

 brane (9) gives support to these cells. 



The walls of the tubules differ little from those of the main 

 duct. The columnar cells are not as tall, but they consist of 

 the same granular protoplasm, with the same large nuclei and 

 the same jagged look at the free edges. They also rest upon a 

 basement membrane, but they show distinct traces of ciliation 

 throughout the whole length of the tube. 



The lumen, which always contains more or less mucus, 

 decreases somewhat in diameter towards the nephrostome, and 

 here the columnar cells are continued in to the flat cells of the 

 pericardial epithelium. In between the tubules are numerous 

 blood-capillaries. As in Bdellostoma, the peripheral tubules 

 are aggregated into lobali, which are invested with pericardial 

 epithelium ; or, again, isolated tubes may be provided with a 

 separate investment (fig. 7). In these cases also the capillaries 

 are always present, lying beneath the epithelium and around 

 the tubes. 



Sections through the pronephros of the other Myxiue 

 present a very different appearance. The anterior tubules for 

 their whole length, and also the distal region of the others, are 

 unchanged, but as these more posterior tubes reach their inner 

 extremity their character is much altered. The lumen is 

 diminished in diameter, or even obliterated, and the tall 

 columnar cells are no longer recognisable : some have become 

 much attenuated — in fact, fibrous, with a dwindled nucleus; 

 whilst others have a greatly swollen nucleus, surrounded with 

 a small layer of protoplasm. A section through this region of 

 a tubule is shown in fig. 8. Within the vein no definite 

 central duct or glomerulus can be made out. The position 

 occupied by these structures in specimens, as already described, 

 is here taken up by a mass of tissue, consisting of a reticulum 

 of protoplasm, whose fibres are nucleated, and in whose meshes 

 are small cells with small nuclei, and larger cells with very 

 large, round, and deeply staining nuclei (fig. 7). Blood- 



