554 EMBRYOLOGY OF THE LOWER VERTEBRATES oh. 



tubular communication between nose and mouth. The lachrymal 

 groove is no longer visible : the lachrymal glands are developing as 

 solid ingrowths of ectoderm. The pituitary body now forms a rounded 

 mass of branched glandular tubes lying between the trabeculae and 

 communicating with the buccal cavity by a narrow tubular duct 

 opening immediately over the glottis. The air-sac rudiments make 

 their appearance on the surface of the lung (Fig. 246, C, a.s). _ 



The mesonephric tubules have been growing actively up till now : 



the metanephric units 

 are making their ap- 

 pearance : the Muller- 

 ian duct reaches the 

 cloaca if it has not 

 already done so 

 although no actual 

 communication is es- 

 tablished until about 

 six months after 

 hatching. 



Ossification be- 

 comes conspicuous in 

 the limb - bones and 

 the investing bones 

 of the head. The 

 keel of the sternum 

 forms an ossification 

 distinct from the two 

 lateral rudiments of 

 the body of the 

 sternum. 



The terminal sinus 

 of the vascular area 

 has disappeared. The 

 septum of the conus 

 is now completely 

 traversed by muscle 

 so that both aortic 

 and pulmonary cavi- 

 ties are completely 

 ensheathed by muscle. The splitting apart of the two vessels is 

 inaugurated by the appearance of a longitudinal incision along the 

 line of attachment of the septum. 



As regards the further progress of development the following 

 approximate times may be mentioned. 



About the ninth day the oesophagus gradually becomes patent 

 again. On the tenth day the arterial arches have practically assumed 

 the definitive condition and the metapodial skeleton is ossified. 



Fig. 244.- 



-Chick extracted from egg during seventh day 

 showing operculum {op). 



