220 EMBRY0L0GICAL TYPES 



ventricle and the systemic arch without going through the right 

 ventricle. The remainder of the blood passes through the right 

 ventricle to the pulmonary arches, and from them through the 

 ductus arteriosus to the dorsal aorta. Little if any goes to 

 the lungs. The interauricular septum is reconstituted after 

 hatching, when the lungs are open and functional, and 

 oxygenated blood reaches the left auricle from the lungs. 

 The interventricular septum is complete. 



Kidneys. — With the exception of the urino-genital organs, 

 the remaining organs of the chick develop in a manner very 

 similar to that which obtains in the frog, and no useful purpose 

 would be served by going over them in detail. The limbs 

 may be mentioned, since they show in their development 

 certain characters which are more primitive than the definitive 

 adult condition ; and especially the urino-genital system is 

 worthy of note. Not only does it differ in some respects 

 radically from that of the frog, but in others it shows certain 

 features more clearly. 



In some half a dozen segments in the anterior region of 

 the trunk, the intermediate cell-mass or nephrotome of each 

 segment pushes out a rod of cells which curves backwards 

 and meets and fuses with the similarly produced rod from the 

 segment next behind it. Each of these rods represents a 

 tubule of the pronephros ; they are solid instead of being 

 hollow and opening into the ccelom by ciliated funnels, because 

 the pronephros is degenerate in the chick, and does not even 

 function as an excretory organ. However, the rod of cells 

 formed by the pronephros on each side in this way grows back 

 to the cloaca, and later becoming hollow, forms the pronephric 

 duct. 



In about a dozen segments, behind the pronephros, the 

 mesonephric tubules develop. Like the pronephros, they are 

 segmental in origin, and as they grow out from the intermediate 

 cell-mass, they find the pronephric duct so to speak ready-made 

 for them. They connect with it which now becomes known 

 as the mesonephric or Wolffian duct. The mesonephric 

 tubules do not as a rule open into the splanchnocoelic cavity ; 

 they have no ccelomic funnels. The tubules give rise by 



