hall: mesonephros and mullefjan duct in amphibia. 39 

 II. Development of the Mesonephros. 



A. AilBLYSTOilA, 



Stage I. 



In embryos of Stage I, the bead and tail are already slightly differen- 

 tiated from the oblong mass of the body. The eye is visible, although 

 sections do not show any sign of the lens. 



About nine body somites are already established, — two in front of 

 the pronephric thickening, two in the region of that thickening, and five 

 posterior to it. In order to make my account more readily comparable 

 with that of Field ('91), I shall call the somites in connection with which 

 the pronephros is developed somites three and four, ignoring the head 

 segments. 



In cross-sections the somites have in outline the form of triangles 

 (Figure 1, Plate 1), the median sides of which abut on the neural tube 

 and chorda. In the middle of the somite there is a lumen. The walls 

 ai-e thick and composed of a single layer of slender columnar cells heavily 

 pigmented in the ends bordering on the lumen. From Figure 1 it will 

 be seen that the lateral plates (so'drm. I. and spTclrm. I.) are consider- 

 ably thinner than the walls of the somite, and that the lateral extension 

 of the lumen of the somite between them is indicated by cell-boundaries 

 only, there being, generall}^ no real cavity. On following the lateral 

 plates ventrad they are seen to become still thinner. 



Frontal sections show the somites as rounded rectangles, somewhat 

 elongated laterally and flattened against each other. Segmentation of 

 the mesoderm extends as far laterad as the point nph^tm. in Figure 1 ; 

 that is, to a point about on a level with the dorso-median boundary of the 

 AVolffian duct {dt. Wf.). The only clue to the position of the mesomer 

 is the pronephric thickening, which, as it is the fundament of the prone- 

 phric tubules, must lie in the mesomeric somatoderm. The section 

 from which Figure 1 was taken passed through the seventh somite. 

 Superposition of camera drawings of that section and those anterior to 

 it shows the pronephric thickening to occupy a position corresponding 

 to that portion of the somatoderm marked nph'im. in Figure 1. How 

 nuich of tlie ventral (splanchnic) wall of the somite is to be assigned to 

 the mesomer, is not to be determined at this stage. The Wolffian duct 

 has reached the middle of the eighth somite. 



