74: Duvelopiiiont and Shape of ('I'iiiircrous Tiiluiles 



to sections of kidneys of pig embryos of these stages a very characteristic 

 appearance, resembling somewhat tlie appearance presented by sections of 

 the mesonephros, and making it easy to distinguish them from sections of 

 kidneys of cat, rabbit, and human embryos of corresponding stages of 

 development. 



The fact that the anlagen of tubules which are first formed are rela- 

 tively far apart, the growth of the tubules developing from them being 

 thus for a time not materially interfered with, may be mentioned as a 

 possible reason for the early development and elongation of the proximal 

 convoluted portions of these tubules, certainly an explanation of the 

 fact that these tubular portions are coiled more in the horizontal plane 

 than similar portions of tubules which develop later. 



Malpighian corpuscles of a stage of development in which they present 

 the form and cellular differentiation shown by these structures in post- 

 foetal life, and proximal convoluted portions of tubules in which the 

 epithelium has become differentiated so as to present the structural ap- 

 pearances presented by it in more fully developed tubules, even though 

 such corpuscles and tubular segments are found in early stages of kidney 

 development, may be thought to have assumed functional activity. If 

 may be suggested, therefore, that in the uriniferous tubules which are 

 first formed, those portions which are especially concerned with the 

 function of excretion are rapidly differentiated to perform this function, 

 while the other portions which are not especially concerned with the 

 function of excretion are developed more slowly. Although there is as 

 yet no unanimity of opinion as to the special functions to be ascribed to 

 the different parts of a uriniferous tubule, assuming that the differences 

 of shape and structure observed in the epithelium lining the different 

 parts of the uriniferous tubules postulates a difference of function for 

 the several parts lined by the especially differentiated epithelium, the 

 generally accepted view is that the Malpighian corpuscles and the proxi- 

 mal convoluted portions of the uriniferous tubules are more particularly 

 concerned with the function of excretion. The assumption, therefore, 

 seems justified that it is for this reason, and not alone owing to mechani- 

 cal conditions, that the Malpighian corpuscles and especially the convo- 

 luted portions of the first-formed uriniferous tubules are developed and 

 differentiated relatively early, so as to enable the permanent kidney, at 

 an early stage in its development, to assume an excretory function. 



Late Stages in the Development of Uriniferous Tubules. 



The further growth and development of uriniferous tubules, after a 

 stage in which all the parts show a characteristic differentiation (as for 



