The development of the urinogenital organs of the lamprey. 41 



the mesonephros. The bilateral structure of the pronephric funnels 

 is copied throughout in the mesonephros and retained even in the 

 adult Petromyson marinus. In cross-section these funnels closely 

 resemble Fig. 36 b of the pronephros. The opening of the funnel, 

 too, is bilabiate, but not so clearly as in the pronephros, since the 

 edges of the nephrostome are not reflected as in the pronephric 

 nephrostome represented in Fig. 36a. The pronephric funnels are 

 often slightly contracted just above the orifice so that they are some- 

 what campauulate. Even this peculiarity (faintly indicated in Fig. 36 a) 

 is often seen in the mesonephric funnels and grows more pronounced 

 with the age of the animal. "Ersatzzellen" very similar to those in 

 the pronephric tubules and funnels are easily found in the mesonephric 

 tubules even in larvœ 17 cm long. The difference in diameter between 

 the anterior and posterior mesonephric tubules is, of course, due to 

 a difference in age; ultimately this difference disappears with the 

 growth of the posterior tubules. 



The principal changes which link the stage of the mesonephros 

 of Ammocœtes represented in Fig. 67 with the adult condition may 

 be briefly described. It was seen that the mesonephros originally lies 

 in the ventral narrow portion of the nephric fold dorsal to the pro- 

 nephric duct into which the tubules open. As the tubules increase in 

 length and become more convoluted they invade successively more 

 and more of the trabecular tissue, and the spaces between them be- 

 come filled with blood-vessels. In Fig. 67 this filling up of the 

 trabecular tissue with tubules and vessels has extended through the 

 ventral half of the nephric lobe. In the dorsal half a few advancing 

 loops are cut. These, too, are soon surrounded by blood-vessels. In 

 the adult Petromysmi marinus the tubules invade the whole dorsal 

 portion of the nephric lobe and increase so greatly in length that 

 their convolutions are closely applied to one another, leaving only 

 very narrow interstices for the blood capillaries and the pigment. 

 This increase in convolution takes place not only in the dorsal portion 

 of the lobe but also in the region between the glomerulus and the 

 duct, for in the adult this region has increased in length so that 

 cross-sections show the glomerulus more dorsal than in Fig. 67, some- 

 what above the middle of the mesial surface of the lobe. In the adult 

 lamprey the tubule consists of three different parts: first, the ciliated 

 funnel, second, the glandular contorted portion of the tubule, and third, 

 an end-piece which opens into the pronephric duct and may be com- 

 mon to several tubules. Several of these end-pieces are cut in a 



