iv MESONEPHKOS 255 



invaginated into the cavity to form a glomerulus {glom) which may 

 become much enlarged so as to extend right out into the splaneh- 

 nocoele. 



As the process of development is traced back into the second 

 region of the mesonephros (stretching approximately from segment 

 16 to 19 or 20) a distinct modification becomes apparent. The inter- 

 mediate cell-mass in this region becomes loosened out into mesen- 

 chyme, and amongst this loose tissue what may be termed the 

 definitive nephrotomes make their appearance secondarily in the form 

 of roundish condensations of cell elements. Each of these becomes 

 more and more sharply marked off from the surrounding mesenchyme, 

 its cells assume a radial arrangement, and presently a small rounded 

 cavity appears in the centre. This cavity dilates and the result is a 

 hollow vesicle with a wall composed of a single layer of cells — the 

 definitive nephrotome. 



In the third, hinder, region of the mesonephros, extending from 

 about segment 20 or 21 backwards, the process in the Fowl, though 

 not in the Duck, has undergone the further modification that the 

 intermediate cell-mass is from an early period completely isolated 

 from the peritoneal epithelium. The peritoneal canals have here 

 completely disappeared, except for faint vestiges, the cells of the 

 peritoneal epithelium still showing here and there traces of the 

 same arrangement as they have further forwards where they are 

 passing into a peritoneal canal. Apart from this separation from 

 the peritoneal lining, the process is similar to that already described. 

 Here also the intermediate cell-mass becomes separated out into 

 loose mesenchyme in which the definitive nephrotomes make their 

 appearance secondarily. 



An important feature in the above described processes of develop- 

 ment is the obliteration of the primitive segmentation of the nephro- 

 tome region. When the definitive nephrotomes become visible, and so 

 bring into view the metameric segmentation of the mesonephros, a 

 further modification becomes apparent in that the mesonephric 

 segments, except towards the front end of the series, are more 

 crowded together than are the primitive mesoderm segments as 

 represented by the myotomes (Sedgwick, 1880). Thus in the Duck 

 Schreiner (1902) found in the region of myotome XX, 4 or 5 meso- 

 nephric rudiments, in that of myotome XXV — 7, in that of XXVI 

 — 9, in that of XXVII as many as 13. 



As development proceeds, the mesonephric elements become still 

 more crowded together inasmuch as from segment 21 or 22 back- 

 wards "subsequent" nephrotomes make their appearance in the 

 mesenchyme. These closely resemble in appearance the primary 

 nephrotomes, with which they are at first in close proximity if not 

 in actual continuity, and they develop in succession one over the 

 other, each series forming a vertical row over its primary nephrotome. 

 The number of subsequent tubules is greatest posteriorly where there 

 are commonly four to a segment. 



