THE DEVELOPMENT OF THE UROGENITAL SYSTEM. 



387 



After the tubules are formed, other condensations of the mesenchyme appear 

 near their inner ends. A branch from the aorta enters each condensation and 

 breaks up into a number of smaller vessels which ramify inside, the entire 

 structure thus becoming a glomerulus. Each glomerulus pushes against the 

 corresponding tubule, the latter becoming flattened and then growing around 

 the glomerulus. In this way the glomerulus becomes surrounded by two layers 

 of epithelium, except at the point where the vessels enter, and the whole structure 

 the Malpighian corpuscle resembles very closely a renal corpuscle of the adult 



Roof Spinal 



plate ganglion Amnion 



Glomerulus 



Mesentery 



Intestine 



Post, cardinal vein 



Mesonephric 

 (Wolffian) duct 



Blood vessel 



Mesonephric 

 (Wolffian) ridge 



Coelom 



Body wall with 

 umbilical vein 



FIG. 345. From a transverse section of a sheep embryo of 21 days (15 mm.), 

 showing the developing mesonephros. Bonnet. 



kidney. Waste products are removed from the blood through the agency of 

 the glomeruli and are carried to the ducts by the mesonephric tubules (Fig. 345). 

 The tubules themselves increase in length and become much coiled. Sec- 

 ondary and tertiary tubules also develop and become branches of the primary. 

 Whether these develop from condensations of the mesenchyme or as buds from 

 the primary tubules has not been determined. Each tubule consists of two 

 parts (i) a dilated part around the glomerulus, composed of large flat cells 

 and forming Bowman's capsule, and (2) a narrower coiled part leading from 



