The development of the urinogenital organs of the lamprey. 37 



nepliric lobes in this stage is that of Fig. 63, which will be under 

 stood without further comment. 



Such is the origin of a mesonephric tubule in Peiromijson, as I 

 conceive it, from a solid ingrowth of the peritoneal epithelium. In 

 reality the conditions are not easy to interpret, and occasionally one 

 meets with sections like Fig. 52, in which the retroperitoneal cells 

 accumulating and dividing in the space between the peritoneum and 

 the duct, might be conceived to give rise to the tubule; but these 

 cells are so very different in character and, moreover, so many cases 

 are seen in which the young tubule is perfectly continuous with the 

 peritoneum, that I cannot accept this interpretation. Even in old 

 Ammocœtes, at the extreme posterior end of the pronephric duct, one 

 finds appearances like that represented in Fig. 68, which seem to 

 admit of only one view, the derivation of the tubule from the peri- 

 toneum. This view becomes still more probable when we examine 

 surface views of the peritoneum and sagittal and frontal sections through 

 the very young tubules, e. g. Fig. 66 taken from a larva 22 mm long, 

 corresponding to the region marked mes. h in Fig. 65, i. e. just behind 

 the last fully formed mesonephric tubule. Here I have represented 

 under a high magnification the edge of the mesonephric fold, containing 

 the pronephric duct (d) seen in optical section through the peritoneum. 

 All the nuclei, but not the cell-boundaries, are represented. At the 

 anterior end of the preparation (to the left) a band of nuclei is seen 

 (mes.b) running parallel with and a little above the duct. This band, 

 which is rather sharply marked off from the remaining peritoneum by 

 the character of its nuclei, is thickened at intervals, and from these 

 thickenings tapering strands of nuclei are seen passing through the 

 retroperitoneal tissue to the duct. Sometimes these strands reach the 

 duct singly, more often two or more of them unite to form a single 

 cord of cells extending to the duct. Further back the band of peri- 

 toneal nuclei — the mesonephric band, as I shall call it — turns 

 down and runs obliquely across, and is continued in the pre- 

 paration, beyond the portion figured, parallel with the lower edge 

 of the duct. Sections show that the left hand portion of the band in 

 the figure is really seen in profile as it lies in a groove, while the 

 right hand portion gives the actual surface view. In this portion the 

 thickenings from which the tubules start are very distinctly seen. 

 They consist of large nuclei around which the smaller, narrower peri- 

 toneal nuclei have a somewhat concentric arrangement. The surface 



