The development of the urinogenital organs of the lamprey. 33 



earliest stages in the development of the pronephros or he would not 

 have denied the metameric origin of the tubules. 



To Semon's erroneous conception of the pronephros of Petro- 

 nif/zon, I shall return in the sequel. 



3. The Development of the Mesonephi'os. 



a) The First Appearance of the Organ and its General 



Characteristics. 



After its complete establishment, the pronephros functions for 

 some time as the only excretory organ of the Ammocœtes. In stage 

 12 (embryos 7 mm long) sections behind the pronephros, like the 

 section represented in Fig. 46, show no indications of a mesonephros 

 in process of formation. Under the chorda, which in this specimen 

 is distorted, are found the two pronephric ducts (d) each abutting on 

 a large posterior cardinal vein. The aorta lies in the median line 

 below the chorda, and a short distance below this vessel and also 

 in the median line lie the huge reproductive cells (gon) which 

 still contain yolk granules. On either side between the aorta and the 

 cardinal vein there are some spaces crossed by bands of cells representing 

 a lymphoid structure, the corpus adiposura. Pigment is deposited in 

 these cells, in the walls of the blood-vessels and in the thin peritoneum 

 which invests the lower surface of the genital organs and the walls of 

 the body cavity. The various structures here described are seen 

 under a higher magnification in Fig. 47, which is taken from a some- 

 what younger embryo. 



No changes leading to the formation of a mesonephros are per- 

 ceptible even in larvœ 8 and 9 mm long (stages 13 and 14), i. e. as 

 long as the larvae remain in the uest, but as soon as they have left 

 the nest and have entered the earth, the mesonephros begins to make 

 its appearance (larvae 12 mm long, stage 15). In the specimen 

 sectioned mesonephric tubules are found from the seventh and ninth 

 segments back of the pronephros on the two sides respectively as far as 

 the twelfth segment on both sides. For a short distance behind the 

 twelfth segment new tubules are forming. This increase in the size 

 of the mesonephros by the addition of new tubules at the posterior 

 end continues from this stage till the animal undergoes metamorphosis 

 (17 cm long). But while the mesonephros is thus increasing at its 

 posterior end it is decreasing at its anterior end. This is shown by 



Zool. Jahrb. XUl. Abth. f. Morph. 3 



