42 WILLIAM MORTON WHEELER, 



single cross-section through the mesonephros of an adult lamprey. They 

 run parallel with one another in the outer ventral portion of the lobe, 

 i. e. on the side next to the myotomes, and they open close together on 

 the outer or lateral surface of the pronephric duct. I believe that 

 there is no increase in the number of mesonephric tubules except at 

 the posterior end of the lobe, so that the filling up of the dorsal 

 trabecular tissue must be brought about by an increase in the length 

 and convolution of the tubules already existing. The number of funnels 

 in the adult is apparently no greater than in the Ammocœtes 9,5 cm 

 long in a correspondingly large piece of the renal lobe. In the tubule 

 cells of the adult lamprey urate bodies are abundant. A web-like 

 structure spanning the lumina of the tubules might be mistaken for 

 cilia but it is, as in the pronephros, merely the residue of the nephric 

 secretion from which the water has been withdrawn by the alcohol 

 used in preservation. The funnels are the only ciliated portion of the 

 tubules. Their cells are deeply columnar and sharply marked oÖ" 

 from the more cuboidal glandular cells of the succeeding portion. 

 These differences are clearly shown in Fig. 59. The end-piece into 

 which the glandular portion gradually passes consists of a somewhat 

 paler and flatter epithelium. The pronephric duct in the adult ap- 

 pears to have lost its former glandular character. In my preparations 

 (hardened in chromic acid) its epithelium stains very faintly like 

 the epithelium of the tubule end-pieces, and the nuclei are scarcely 

 visible. 



d) The Vascular System of the Mesonephros. 



In concluding the description of the mesonephros we may con- 

 sider the blood-vessels of the organ , beginning with the conditions 

 seen in Fig. 46 and 47 where we have only the dorsal aorta and the 

 two posterior cardinal veins with some irregular branches in the 

 trabecular tissue of the nephric lobes. All three of these vessels in 

 this stage are sharply outlined, although their endothelial walls are 

 not always apparent. These I beUeve to arise as in the case of the 

 other blood-vessels of the lamprey from migrating amœboid blood- 

 corpuscles, which attach themselves to the walls of a simple cavity in 

 the mesenchyme and finally flatten out and become united with one 

 another at their edges. In Fig. 47 the endothelial walls are not yet 

 established in the posterior cardinals and are only beginning to form 

 in this portion of the aorta. 



In larviB 12 — 15 mm long the three vessels have taken on the 



