82 



Devc'lopinont and Slia])o of T^i'iiiifcrous Tubules 



twice that of the greater part of the deseeiiding linil) and are lined by a 

 cubical epithelium. The ascending limb reaches the coil complex in 

 the immediate vicinity of the Malpighian corpuscle with which it is 

 practically in contact for about one-third of the hitter's circumference. 

 The proximal limb leaves the coil complex in close relation with the 

 upper end of the distal limb, therefore very near to the Malpighian cor- 

 puscle. The lower end of the loop of this tubule reaches to the pelvis 

 of the kidney, being separated from its epithelium by only a small 

 amount of interstitial tissue. 



In the tubules shown respectively in B of Fig. 19 and in Fig. 20, the 

 Malpighian corpuscle is fully developed and the proximal and distal con- 

 voluted portions may be regarded as also fully developed and as having 

 attained the size and for all practical purposes the length to which the 

 tnlnilar portions grow. This statement is based on the measurements 

 made on nriniferous tubules — especially those which are developed earli- 

 est — at different stages of their development. The measurements given 

 below show that the proximal convoluted portions at a relatively early 

 stage in their development attain a length which is about that presented 

 by these tubular segments in later stages of embryonic development or 

 at birth. Tlie following summary of certain of the measurements pre- 

 viously given presented in the form of a table may serve to substantiate 

 this : 



Tubule. 



Fig. 16, A 

 Fig. 16, B 

 Fig. 20 



Fig. 14, D 

 Fig. 19, A 

 Fig. 19, B 



Obtained fkom. 



Cat embryo, 4 cm. 

 Cat embryo, 6.5 cm. 

 Cat embryo just before 



birth 

 Rabbit embryo, o.5 cm. 

 Rabbit embryo, 6.5 cm. 

 Rabbit at birtli 



2.9 mm. 

 5.8 mm. 

 7.8 mm. 



z o 



3.75 mm. 1.9 mm. 

 5 mm. 2.05 mm. 



9 mm. 2.4 mm. 



,1.5 mm. 

 2.2 mm. 

 1.85 mm. 



,1.1 mm. 



.55 mm. 



Loop. 



.75 mm. 

 1.50 mm. 

 4.85 mm. 



2.75 mm. 



4.8 mm. 



The data given in the table will show, I believe, that >the elongation 

 of the nriniferous tubules, after a certain period in their development, 

 is to a large extent due to a growth in length of the loop of Henle, the 

 proximal convoluted portion of each tubule attaining at a relatively 

 early period in its development approximately the length shown by this 

 tubular segment in fully developed tubules. 



To show how much of a single uriniferous tubule, representing a stage 

 in which the ^lalpighian corpuscle and proximal and distal convoluted 



