Embryology. 23 



time, have grown from the front end of the intestine just back of the 

 gills. They begin as a single little sac or bladder growing from the 

 underside of the intestine, i This divides into two which take their 

 places under and on each side of the intestine. The short connection 

 that at first joined the lung sac to the upper intestine, first lengthens 

 and then divides into two branches, one going to each of the two sacs 

 (after their separation). The upper end of the intestine to which the 

 lung tube is joined, finally develops into the pharynx, and the lung 

 tube differentiates at its upper end into the larynx, and at its lower into 

 the wind pipe, or trachea, while the branches into the lungs are called 

 the bronchial tubes. The development of the lungs keeps pace with 

 that of the heart, so that by the time the heart has become four cham- 

 bered the lung circulation has become complete and separate from the 

 body circulation. 



Very early in the development of the embryo begins the formation of 

 the primitive kidneys, or wolffian bodies, as they are called. The use 

 of the kidneys is to. absorb and carry out of the arterial blood excessive 

 quantities of nitrogenous matters and water. The primitive kidneys are 

 supposed to be developed from the skin sensory layer. At first they 

 are a mere string of cells set off just under the outside skin of the back 

 on each side of the rudimentary vertebral 4 column. These strings en- 

 large and become a pair of tubes reaching from front to rear. During 

 the development of the embryo, these tubes, or kidney ducts, as they 

 are now called, are crowded inward and finally bring up on the inner 

 surface of the body cavity outside of and above the intestine. In the 

 meantime they develop on their inner and under side, a row of short 

 tubular branches. These receive the blood vessels from which the prim- 

 itive urine is extracted. 



The wolffian bodies disappear in the ninth or tenth week in human 

 pregnancy, and their functions are assumed by the permanent kidneys 

 and renal capsules. In the other mammals the wolffian bodies persist 

 for a longer time, and in the fishes and amphibians they are the per- 

 manent urinary organs during life. 



The posterior end of the kidney tube opens into the rear end of the 

 intestine, which then forms a cloaca, or common receptacle, as in the 

 monotremes, birds, reptiles and fishes. From this cloaca, in the young 

 embryo, the opening is into the allantois. The kidney tubes, with 

 their tubular appendages, extend forward to the region of the heart. 



From the posterior section of the primitive kidfty duct the secondary 

 or permanent kidney ducts originate at an early period. The secondary 

 or permanent kidney duct grows out from the primitive kidney duct 

 near its entrance into the cloaca. It therefore originates from the skin 

 layer, as does the kidney. It grows forward as a tube and gives rise to 



