Development 0/ Gastrosteus spinachia. 489 



motions of the fish, the snout being introduced and worked 

 about until pocket-like cavities are formed, or the creature, 

 as often happens, passes and repasses through the cavities 

 with similar results. The thread-like material which binds 

 the nest is a remarkable product. It is secreted by the male, 

 is colourless, tenacious, of the consistency of mucilage when 

 freshly extruded, and exhibits a delicate blue opalescence 

 which disappears in two or three days, leaving the threads of 

 a transparent grey or dirty- white colour. According to Mobius 

 it is nitrogenous, as is shown by treating with acids and 

 alkalies, and evidently a form of mucin peculiarly modified. 

 Like normal mucin it is gelatinous and viscid in water, turn- 

 ing white like tallow on immersion in spirit. Carmine stains 

 it deeply. 



On examining the male at the breeding-season, the kidneys 

 are seen to be considerably swollen, the enlargement being 

 especially noticeable posteriorly (fig. 1 a) . Sections of 

 the kidneys reveal an altered condition of the sinuous tubules 

 (fig. 1 A, &), the conical epithelial cells of which are swollen at 

 their free ends and indefinite in outline. The nucleus of each 

 cell is slightly displaced and occupies a more terminal posi- 

 tion than in the normal condition. These epithelial cells are 

 active in secreting the material used in constructing the nest. 

 They perform the function, indeed, of cell-glands, and their 

 secretion is carried by the uriniferous tubes to the outer ven- 

 tral border of each kidney, where a large duct passes longi- 

 tudinally. In cross section the ureters (PI. XIV. fig. 1 A, a, a) 

 are oval, and their capacity is very great at this time, the 

 walls being of dense fibrous tissue lined with pavement 

 epithelium. Both ureters emerge from the renal mass near 

 the posterior end and^ descending in a forward direction, be- 

 come applied to the wall of the so-called urinary bladder, which 

 at this point is somewhat attenuated, and, passing anteriorly, 

 they open obliquely from without inwards into the bladder. 

 This structure, it is unnecessary to say, is not morphologically 

 connected with the urinary receptacle of higher Vertebrates, 

 the lengthened course of the ureters, of which it is simply 

 a dilated common portion, being due to its extraordinary de- 

 velopment in the male stickleback. In a fish 5^ inches in 

 length it is about an inch long, and at its widest part -1 inch 

 in diameter. Situated on the right side of the abdominal 

 cavity, immediately below the swim-bladder in the post- 

 hepatic region, it has the form of a capacious pyriform 

 sac, ending blindly anteriorly, and diminishing in circum- 

 ference as it passes backwards (PI. XIV. fig. 6, a). Be- 

 fore terminating posteriorly it describes a double curve, 



