VI.] THE WOLFFIAN BODY. 193 



differentiated out of the mesoblast of the intermediate 

 cell-mass and open independently into the Wolffian 

 duct. 



A tubule of the Wolffian body typically consists of the follow - 

 ing parts, (1) a section carrying the peritoneal opening, and 

 known as the peritoneal funnel, (2) a dilated vesicle into which 

 this opens, (3) a coiled tubulus proceeding from (2), and termi- 

 nating in (4) a wider portion opening into the Wolffian duct. 



In the chick, the peritoneal funnel is only found in the most 

 anterior tubules and soon atrophies; it is not developed in the 

 tubules of the posterior part of the Wolffian body. Region No. 

 4 also is not clearly marked off from region No. 3. One part of 

 the wall of the dilated vesicle (2) is invaginated by a bunch of 

 capillaries and gives rise to the Malpighian body. 



In consequence of the continual folding in of the 

 somatopleure and especially of the splanchnopleure, as 

 well as owing to the changes taking place in the meso- 

 blastic somites, the Wolffian duct undergoes on the 

 third day a remarkable change of position. Instead of 

 lying, as on the second day, immediately under the 

 epiblast (Fig. 34, W.d), it is soon found to have appa- 

 rently descended into the middle of the intermediate 

 cell-mass (Fig. 64, w.d) and at the end of the third day 

 occupies a still lower position and even projects some- 

 what towards the pleuroperitoneal cavity. (Fig. 65, 

 W.d.) 



The chief events then which take place on the third 

 day are as follows : 



1. The turning over of the embryo so that it now 

 lies on its left side. 



2. The cranial flexure round the anterior extremity 

 of the notochord. 



F. &B. 13 



