416 DISCOVERY REPORTS 



capsula and in intimate contact with it Daudt described the tunica albuginea, a third 

 fibrous investment, extending into the organ between the lobes and lobules of which it is 

 composed. 



In the youngest kidney examined in the present work — that of a B. musculus foetus 

 18-5 cm. in length (Fig. 23 /) — only one fibrous layer could be seen in section, completely 

 surrounding the kidney and sending branches into the interior of it. Bouvier (1892) 

 mentions only one fibrous capsule round the kidney of Hyperoodon. In Balaenoptera 

 this fibrous capsule is derived partly from the peritoneum and partly from the thick 

 fibrous pad situated immediately dorsal to the kidney between the kidney and the 

 hypaxial muscle. In kidneys from foetuses of length 0-5-1-0 m. this enveloping serous 

 coat can be seen to be composed of two layers separated by lacunae (Figs. 24, 25), which 

 become progressively more extensive in older kidneys. These form the reticulate 

 peripheral venous channels to be described later and their uniform distribution through- 

 out the serosa divides it into an outer fibrous layer, the true serosa, deriving most of 

 its fibres from the peritoneum, and an inner layer, the capsula fibrosa, which derives 

 most of its fibres from the fibrous pad above mentioned. The two layers, although con- 

 tinuous to a large extent, can be dissected apart with ease in kidneys of foetuses more than 

 about 1 -o m. in length. In the adult the space between these two investments is abundantly 

 charged with fat so that the serosa is thrown into folds and ridges of fatty tissue which 

 mask the true shape of the kidney. The capsula sends fibrous septa into the body of the 

 organ between the lobes and lobules of which it is built up. Under the capsula the lobes 

 and lobules of the young kidney are extremely closely invested by a very thin layer of 

 fibres which is inseparable from them and extends among the capsules and tubules and 

 forms a scaffolding within the kidney tissue. This probably represents the tunica 

 albuginea of other mammals, since in older foetal and adult kidneys it forms the covering 

 of the individual "renculi" — little kidneys — of which the organ is composed. 



The serosa forms a connective tissue sling in which the kidney is suspended within 

 the body cavity. In young foetuses up to about 0-5 m. in length the serosa of the kidney 

 is almost flush with the peritoneum forming the roof of the body cavity — that is to say, 

 the kidney does not hang down within the cavity, but lies under the connective tissue 

 layers of the roof, a slight swelling only marking its position. Later, however, it becomes 

 slung from the roof of the body cavity as it increases in size and weight, and the part of 

 the peritoneum which formerly covered it becomes the serosa, attaching the kidney to 

 the body wall along a broad strip following the angle between the latero-dorsal and mesal 

 faces. The line of attachment of the serous mesentery to the kidney runs from the point 

 of entrance of the blood vessels into the mesal face, about one-third of the length of the 

 organ from the anterior end, to the point of emergence of the ureter behind. As 

 mentioned before, the attachment of the serosa to the body wall is closer posteriorly 

 than elsewhere and causes a lateral deflection of the ureter. Anteriorly the line of attach- 

 ment shifts on to the mesal face itself so as to include the blood vessels, and in front of 

 them the serosa extends on to the diaphragm. The serosa of the left kidney has also an 

 attachment to the ventral abdominal wall, as shown in Fig. 22, throughout a portion of 



