44 Development and Shape of Uriniferous Tnbules 



cavity and changing it from a structure having the shape of a segment 

 of a sphere to a hemisphere, the mesenchyme found in the concavity of 

 the shallow structure of the earlier stage proliferating as this obtains a 

 greater concavit}^, especially noticeable being an increase in the number 

 of the capillary loops. That the epithelial portion of this structure is 

 double-walled is evident from what has been said of its anlage and further 

 development; its border represents the edge along which the outer wall 

 becomes continuous with the inner wall. At about this stage of its de- 

 velopment, its inner layer on the side toward the collecting tubule and 

 in the region where it is continuous with the tubular portion of the 

 anlage and at about the level of its border, develops a fold which grows 

 outward, away from the collecting tubule, which fold in its further growth 

 becomes continuous with the border. In sagittal sections, this fold ap- 

 pears as a spur and is seen in F and of Fig. 3 and also in certain of 

 the tubular anlagen as shown in Fig. 11. It assists in narrowing the 

 wide opening by means of which the space (occupied by mesenchyme 

 and capillaries) enclosed within the double-walled structure communi- 

 cates with the outside. This fold further assists in differentiating more 

 clearly this structure of hemispherical shape from the tubular portion 

 of the anlage, in that the former now obtains a continuous border along 

 the entire length of which the inner wall is reflected into the outer wall. 

 This brings the attachment of the tubular portion in connection with its 

 outer wall. The attachment of the tubular portion is always on its inner 

 side, on the side turned toward the collecting tubule. 



In a tubular anlage developed to the extent here described, there may 

 be recognized the anlagen of the different parts of a uriniferous tubule, 

 as these are known after its development. Briefly stated, the genesis 

 of the different parts of a uriniferous tubule is as follows : The double- 

 walled hemispherical structure, which develops from the greater part of 

 the lower S-curve, forms the inner and the outer layer of Bowman's 

 capsule (more correctly stated, the outer layer forms the epithelial lining 

 of Bowman's capsule, while the inner layer forms the glomerular epithe- 

 lium) ; the space between the two layers, continuous with the lumen of 

 the tubules, forms Bowman's space; the vascular mesenchyme enclosed 

 within this double-walled structure forms the anlage for the glomerulus, 

 the two structures taken together forming a stage in the development of 

 a Malpighian body or corpuscle. The region of the junction of the 

 lower S-curve and the S-middle piece, which, as stated, differentiates 

 into an arched tubular segment, represents the anlage of the proximal 

 convoluted tubule. Between this arched tubular segment and Bow- 

 man's capsule there is described a short segment, which is known as 



