THE EMBRYO. 



1207 



off the greater part of the thoracic from the abdominal cavity, but posteriorly 

 there remain two channels of communication, one on each side of the alimentary 

 tube ; these channels subsequently become the pleural cavities, and are shut off 

 from the abdomen by folds which grow from the lateral and posterior parte of 



( Stroma 



Genital J of ovary.' 

 ridge. I Primitive _\ 

 ova. 



Mesentery.--*' 



Wolffian duct. 



_ . Miillerian duct. 



----.Wolffian 

 -~" tubules. 



-Body wall. 



FIG. 774. Section of the urogenital area of a chick embryo of the fourth day. (Waldeyer.) 



the trunk and which fuse with the posterior edge of the septum transversum. 

 Sometimes the fusion is incomplete, thus leaving a permanent communication 

 between the abdominal and one or other of the pleural cavities, and through 

 which some of the abdominal contents may pass, forming what is termed a 

 diaphragmatic hernia. 



Development of the Urinary and Generative Organs. The urinary organs are 



FIG. 775. Enlarged view from the front of the left Wolffian body before the establishment of the distinc- 

 tion of sex. (From Farre, after Kobelt.) a, a r b, d, Tubular structure of the Wolffian body; e, Wolffian duct; 

 /, Its upper extremity ; g, Its termination in a-, the urogenital sinus ; h. The duct of Miiller ; i, Its upper, funnel- 

 shaped extremity ; k, Its lower end, terminating in the urogenital sinus; I, The mass of blastema for the repro- 

 ductive organ, ovary or testicle. 



developed from a ridge of mesoblast at the point where this layer separates into 

 somatopleure and splanchnopleure. As this ridge is situated close to the epiblast, 

 between the paraxial mesoblast and the common pleuro-peritoneal cavity, it has 

 been named the " intermediate cell-mass." It is at first solid, and in it is formed 

 a cord-like arrangement of some of the cells, which extends longitudinally from 

 just below the heart to the posterior extremity of the body-cavity. In this cord- 

 like structure a tube is hollowed out; it gradually becomes separated from the rest 

 of the intermediate cell-mass, and is then named the Wolffian duct (Fig. TOO). 1 Its 

 posterior end becomes connected with and eventually opens into the hind-gut. Its 

 anterior end becomes connected with pit-like involutions of the peritoneal epithelium, 

 and in the mesoblastic tissue between these invaginations a vascular glomerulus is 

 formed which projects into the peritoneal cavity. It is known as the head-kidney 

 1 By some embryologists the Wolffian duct is regarded. as being of epiblastic origin and formed 

 by a longitudinal invagination of the epiblast. 



