THE ANATOMY OF POLYPTEEUS. 337 



kidney. The section shows also the similar appearance of the testis-duct and 

 ureter. The uriniferous tubules of the kidney are embedded in a mass of 

 lymphoid tissue. 



Fig. 17. A similar section passing behind the testis (p. S25). When compared with 

 fig. 15 the figure shows the relatively great development of the main duct 

 of the testis-ridge, the small development of the ductules of the ridge in 

 number and size, and the small relative size of the ureter. 



Fig. 18. A similar section in the region of the urinogenital sinus (p. 325), showing the 

 opening of the vas deferens into the latter and its lining of large columnar 

 cells. The large relative size of the vas deferens is seen on comparing with 

 fig. 14. 



PLATE III. 



Fig. 19. A vertical section through the testis and ureters of a very young male 

 P. senegalus, 9 cm. in length (p. 325), showing the precocious development of 

 the testis-duct, the tubules of the testis not being yet formed. 



Fig. 20. A similar section through the posterior termination of the testis-ducts (p. 325), 

 showing that they are only just acquiring their connection with the urino- 

 genital sinus. 



Fig. 21. A vertical section through the developing ovary and ureter of a very youno- 

 female P. senegalus, 9 cm. in length (pp. 325, 326, 327), showing the ova 

 developing from the external wall of the ovary, the ovary divided into a 

 number of loculi, and the artery and vein in the median walk 



Fig. 22. A similar section behind the ovary passing through the peritoneal opening of 

 the oviduct (pp. 325, 326). 



Fig. 23. A similar section behind the peritoneal opening of the oviduct (pp. 325, 326), 

 showing the oviduct lying between the mesoarium or female genital ridge 

 and the ureter. To be compared with fig. 17. 



Fig. 24. A similar section through the posterior termination of the oviduct (p. 326), 

 showing that they have not yet acquired their connection with the urino- 

 genital sinus. To be compared with fig. 20. 



Fig. 25. Left operculum bearing external gill of Polypterus lapradei, 30 cm. in length 

 (pp. 331, 332), showing the blood-supply to the gill and the efferent artery 

 giving off a branch which runs parallel with the afferent artery. The 

 muscles of the operculum and the gill are also shown. 



Fig. 26. An enlarged drawing of the tip of the external gill (pp. 331, 332), showing 

 its minute structure and the continuation of the afferent into the efferent 

 artery at the end of the gill. 



Fig. 27. A pinnule of the external gill highly magnified (p. 331). 



