ment of the Kidney, and the Development of the - Journal of Cell ...

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450 PETER GRAY in Text-fig. 1, H. As blindly ending tubules (BT., Text-fig. 1, E) are occasionally found scattered among the developing units and collecting-trunks in the posterior region, it is probable that such anastomoses as there are have been produced by the attachment of these blind tubules rather than by fusion between the trunks themselves. Moreover, it may be observed that there is no spare blastema surrounding the archinephric duct in the adult, and it seems probable that such blastema as remains, after the formation of collecting-trunks and units has ceased, may be used up in the formation of numerous little accessory collecting-trunks. That such an anastomosing mass exists is shown by the work of Stewart (1927), who records that 'under conditions of injection and dissection of the ramifications of the collecting duct tree, it was not possible to make out in many instances the junction of the collecting duct and its distal convolutions. It is probable that there are many more than seven orders in the collecting duct system'. His figures show clearly that he was working upon a collecting-duct of the mid or posterior region. The existence of the straight tubule and its connexions is very clearly seen in sections of the sexual region. Fig. 7, PI. 20, represents a section across the extreme anterior end of the kidney of a second-year frog. The straight tubule (st.) ends in the abortive unit (au.). This most anterior of straight tubules is without excretory connexions. Fig. 6 on the same plate is from about 3 mm. farther back in the same series; the straight tubule (si.) is still equally distinct but shows quite clearly the attachments (ae.) of two functional units. At this period there is no trace of collecting-trunks. Collecting-trunks are best seen in sections of a third-year kidney. Three sections from the same series of a 53-mm. frog are shown on Plate 20. The most anterior of these, fig. 5, PI. 20, is from the middle region. The straight tubule (st.) is not nearly so sharply differentiated as it was in the two-year frog, but can still be distinguished from the surrounding tissues. The attachments (ae.) to the functional units are best seen in the enlarged central portion of this figure reproduced as fig. 17, PI. 22. By this time, however, collecting-trunks have
DEVELOPMENT OF AMPHIBIAN KIDNEY 451 appeared and show as small ducts (ct., fig.5, PL 20) with deeply staining nuclei. Fig. 1, PL 20, is taken towards the hinder end of the kidney in the region where the archinephric duct (ad.) is just passing away from the main mass. The last collectingtrunk to be directly connected is shown leaving the archinephric duct and is seen to be surrounded by minor collectingtrunks (ct). Several developing glomeruli (dm.) are cut in this section. The last section (fig. 2, PL 20) is taken through the true posterior kidney, well behind the point of separation of the archinephric duct. Even at this late age (three years) the tissues still consist largely of blastema in which many developing units appear. The collecting-trunks (ct.) which run backwards from the more anterior point of attachment can again be clearly differentiated by their histological structure. The gradual loss of distinctness which is noticeable in the straight tubule is well illustrated by fig. 3, PL 20. This is through the middle region of a 42-mm. (second-year) frog. The straight tubule, which here serves solely for the attachment of excretory units, is beginning to coil in the manner indicated in Text-fig. 1, so that it appears cut in several places. It is more readily distinguishable from the surrounding excretory tissues than will be the case a year later (fig. 5, PL 20), but is markedly less obvious than it was a year before (figs. 6 and 7, PL 20). To sum up, then, the methods of unit attachment shown in the post-metamorphic kidney, there are, passing from anterior to posterior: (1) One, or at the most two, anterior straight tubules devoted to the carrying of sperm. (2) A further series of three or four straight tubules which carry both sperm and excretory products. (3) About six 'straight tubules', later becoming bent, which carry only excretory products. (4) A number of posterior collecting-trunks which are produced irregularly and anastomose among themselves. (ii) Production of Accessory Peritoneal Funnels. It was noted in Part I of this investigation (this Journal, vol. 73, pp. 533-7) that there are two methods for the production
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