426 DISCOVERY REPORTS 



The most perfectly formed convoluted tubules seem in the youngest stages to be 

 coiled in an irregular manner (Fig. 30 A), influenced apparently by the amount of space 

 available among the surrounding tubules. They are lined by large cubical granular 

 cells with deeply-staining round nuclei and abundant protoplasm. There is a narrowing 

 between the capsule and the tubule, and the cubical epithelium lining the tubule is con- 

 tinued for a short distance into the capsule before it merges with the flat epithelium 

 lining the latter. Between the tubules run intertubular fibrous strands of connective 

 tissue continuous with the interlobular septa. 



(ii) Two or more tubules may join together to form a collecting tube, or the tubule 

 may have its own independent collecting tube leading into the main trunk of the 

 lobule (Fig. 30 ^, B). 



The collecting tube system of the lobule at this stage (foetuses about 0-2 m. long) 

 usually exhibits a main trunk running inwards towards the ureter from the periphery 

 (Figs. 27 g, 30 A). It may be joined by one or two main trunks during its course and 

 these main tubes are joined by subsidiary ones from the tubules (Fig. 30 A). As 

 already stated, in older kidneys, in which the lobule has split up into component 

 renculi, the collecting tubes converge to a common focal point (Fig. 30 B) which be- 

 comes the papilla of the renculus ; the point of junction forms a cup-shaped pelvis 

 leading into the common calyx (tertiary duct), which in turn passes from the renculus 

 to the secondary duct. At the periphery of the lobule or of the young renculus, the main 

 trunk of the collecting tube enters the embryonic area as will shortly be described, and 

 forks dichotomously once or twice before ending blindly (Fig. 30 A, B). 



The cells of the collecting tube have large clear round nuclei, and less protoplasm in 

 proportion to the size of the nucleus than is found in the cells of the tubular epithelium. 



(iii) The blind processes of the collecting tube system (Fig. 30) reach the periphery 

 of the lobule (in very young foetuses) or of the young renculus (foetuses about 1 -o m. 

 long) within what will be called the " embryonic area" of the lobule or renculus. These 

 embryonic areas (Figs. 23 g, 27 d) are patches of more or less indifferent tissue where 

 new capsules and tubules are formed. The existence of an embryonic area appears to be 

 called forth by the approach towards the periphery of the blind ends of the dichoto- 

 mously branched collecting tubes (Figs. 28, 29, 30). The tubes appear to cause around 

 their extremities the formation of a mass of deeply staining cells with the potentiality of 

 developing into a capsule or capsules (Figs. 28 A 1 , A 2 , B, 29 B). Where the collecting 

 tubes approach the periphery a basin-shaped proliferation of tissue is formed around its 

 tip (Figs. 28 A 1 , A 2 , 29 B, 30 A, B). Frequently, owing to the proximity to one 

 another of two branches of a fork, two basin-shaped thickenings are formed, joined by 

 their edges (Figs. 28 A 2 , 29 B). Similar thickenings also seem to occur wherever a 

 collecting tube passes on its course near the periphery of the lobule (Fig. 28 A 1 ). Thus, 

 at the earliest stage of peripheral differentiation of the lobules (foetuses about 0-2 m. 

 long), the embryonic patches are found distributed around the inner and outer 

 margins of the crescentic cross-section of kidney substance. Later, when the lobules 

 have become more completely separated as architectural units, the embryonic areas 



