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Part III. — Eleventh Annual Report 



From this aggregation of cells the pronephros is developed as a fold 

 (tig. 19, prn.), extending outwards and upwards between the remainder 

 of the intermediate cell-mass and the tissue separating it from the yelk 

 beneath. The fold is solid, and is formed of a double row of cells very 

 like a solid pouch. More posteriorly (fig. 18, prn.), the fold has a lumen 

 at its ventral and inner end, and the pronephric tubule communicates 

 freely with the body cavity. While the pronephric funnel is shown on 

 the right hand side of the drawing, the solid well-defined rod is seen on 

 the left. The fold, which in fig. 19 was a longitudinal aggregation of 

 cells, becomes in fig. 18 an oval mass of radially arranged cells. The 

 organ is first formed in the mid-region of the trunk, and so gradually 

 extends forward. Whether the pronephric funnel, shown in fig. 18 com- 

 municating with the body cavity, is the same as those shown in figs. 27 

 and 28, opening into the cavities lodging the glomerulus, I am unable 

 positively to affirm. From a study of sections of older embryos, I am 

 inclined to the belief that the opening persists. At any rate, in the em- 

 bryo of fig. 18, though there is a solid fold extending through several 

 sections, there is only a single nephrostome confined to two sections. 



The pronephric fold grows rapidly forwards and also extends backwards. 

 Posteriorly, in embryos of the ninth day, it is developed from the same 

 intermediate cell-mass, bub more from its ventral and outer margin. Fig. 

 21 shows it as a solid blastema on the left, and on the right of the figure 

 the radially arranged cells of the rod have separated and formed the lumen 

 of the duct (s.d.). Fig. 17 is a section of an embryo, thirteen hours older 

 than that of fig. 21, and shows the organ prn. as a well-marked ventral 

 rod of the intermediate cell-mass. 



Till about the tenth day the ducts are on the same level as the upper 

 half of the gut-wall, but as the embryo advances they become more dorsal 

 (figs. 5, 6, 22, 23 s.d.), and at birth (fig. 7) they migrate nearer the 

 middle line, nearly underneath the notochord. 



The cells bounding the lumina of the ducts are in a single layer, and 

 regularly cylindrical. The ducts run parallel with each other till they 

 unite near the anal portion of the gut, and open by a single median duct 

 into the widely dilated portion or urinary vesicle (figs. 24 and 25, u.v.). 

 The urinary dilatation, which is formed very early, opens, as M 'In tosh 

 and Prince* demonstrated in Molva vulgaris (Cf. Plate XX. fig. 13), into 

 the rectum. Listf describes a similar connection of rectum and urinary 

 bladder in Crenilabrus pavo. This communication of urinary bladder and 

 rectum is shown in fig. 26, which is a drawing from an embryo of the 

 fourteenth day. The cylindrical character of the cells of the tube is main- 

 tained till they end in the widely dilated urinary vesicle, but the walls of 

 the vesicle itself are formed of flattened epithelium, the cylindrical cells of 

 the ducts merging gradually into the flat cells of the thin wall of the 

 vesicle. After the birth of the embryo the posterior extremities of the 

 ducts are continued still further backwards, so that they form an elbow- 

 shaped bend before joining the urinary bladder. The urinary vesicle after 

 the extrusion of the embryo from the enveloping membrane has no longer 

 any connection with the hind-gut ; it is prolonged ventrally alongside of 

 the papillary elevation of the rectum, but posterior to it, and ends blindly 

 at the same level as the still closed anal termination of the intestine. Two 

 stages in the downward development of the urinary bladder are shown in 

 figs. 24 and 25. In both cases the vesicle is irregular in outline, but this 

 may in part be due, though not wholly, to the unequal shrinkage brought 

 about by the action of the reagents used in manipulation. 



* Loe. cit. 



t ' Ueber die Beziehung der Harnblase zu den Enddarme bei Teleostierembryonen 

 (Labriden), Anat. Anzeigcr, iv. Jahrg. No. 16. 



