OH. LVIII.] 



DEVELOPMENT OF THE LUNGS. 



blood-vessels, cartilaginous rings, and muscular fibres of the 

 bronchi from the mesoblast. 



The diaphragm is early developed as a partition of mesoblast 



Fig. 674 illustrates the development of the respiratory organs. A, is the oesophagus of a 

 chick on the fourth day of incubation, with the rudiments of the trachea on the lung 

 of the left side, viewed laterally ; i, the inferior wall of the ossophagus ; 2, the upper 

 portion of the same tube ; 3, the rudimentary lung ; 4, the stomach ; it, is the same 

 object seen from below, so that both lungs are visible, c, shows the tongue and 

 respiratory organs of the embryo of a horse ; i, the tongue; 2, the larynx ; .1, the 

 trachea; 4, the lungs, viewed from the upper side. (After Rathke.) 



dividing the original pleuro-peritoneal cavity into thoracic and 

 abdominal serous cavities. 



Development of the Grenito-urinary Apparatus. 



In the early stage of the development of the urino-genital 

 organs, the most striking thing seen is their resemblance to the 



Fig- 675. Diagram of transvene section of embryo dogfish. On the right of the middle 

 line, A B, the primitive segmental tube (A') is seen in transverse section ; on the left 

 side a later stage is represented ; it heie forms a well marked projection into the 

 pleuro-peritoneal cavity, and the section is represented as passing through the 

 trumpet-shaped opening of the tube into that cavity (A"). 



segmental organs, or nephridia of worms. The subject was first 

 worked out by Balfour in the elasmobranch fishes ; we may there- 

 fore first describe what he found here, and then pass on to what 

 occurs in mammals. 



