The development of the urinogenital organs of the lannprey. 35 



latter section first and comparing it with the condition in Fig. 46, 

 we observe that the renal organ consists of two dependent lobes, one 

 on either side of the mid-dorsal line. Each lobe contains in its base 

 the much enlarged posterior cardinal vein, full of blood-corpuscles, 

 and in its apex the pronephric duct. The adipose tissue, poorly 

 developed in the younger stages, has increased considerably and is 

 traversed by blood vessels, some of which run from the cardinal 

 veins as far as the pronephric ducts. Indeed, these small vessels al- 

 ready envelop the ducts. It is certain that the principal agent in 

 causing an enlargement of the renal folds is an increase in the fatty, 

 lymphoid tissue and that this enlargement precedes to some extent 

 the development of the mesouephros. Two successive stages in the 

 formation of a tubule are seen in the two mesonephric lobes in Fig. 63. 

 On the left side below and mesial to the pronephric duct the deeply 

 staining cells of the peritoneum are aggregated to form a small 

 swelling which juts out slightly into the body cavity. A sharply 

 defined membraua limitans separates these cells from the retroperi- 

 toneal trabecular tissue, especially in this larva which was hardened 

 in Pekenyi's fluid. The next stage in the development of the tubule 

 is shown on the right side of the same figure as a peg-shaped mass 

 of deeply-staining cells extending from a thickening in the peritoneum 

 to the outer surface of the pronephric duct. 



For the purpose of elucidating the origin of the mesonephric 

 tubule, I have represented in Figs. 52 to 57 a series of tubules in 

 different stages of growth taken from a larva 15 mm long (stage 16). 

 To these Fig. 58 may be added, taken from the same larva as Figs. 63 

 and 64. Fig. 52 represents the first recognizable trace of a tubule 

 in the form of a thickening of the peritoneal epithelium (mes) even 

 less pronounced than that represented in the left mesonephric lobe of 

 Fig. 64. Below this thickening are seen a number of cells with pale 

 rounded nuclei, occasionally, as in this particular instance, dividing by 

 mitosis. The next stage (Fig. 53) shows that the peritoneum has in- 

 creased greatly in thickness and that there is a conical mass of 

 similar cells extending from this point to the wall of the pronephric 

 duct. In a slightly older stage (Fig, 54) the cord of cells has grown 

 thicker, especially at the end which is appHed to the wall of the duct. 

 The karyokinetic figure bears witness to the method whereby this 

 growth is accomplished. In this and the previous sections there is 

 a line of demarkation between the cells of the cord proper and the 

 peritoneum, but this is not visible in all cases at so early a stage. 



3* 



