554 EMBRYOLOGY OF THE LOWKU VERTEBRATES < n. 



tubular communication lu-tueen nose and mouth. The lachrymal 

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

 solid ingrowths of ectoderm. Tin- pit uitary 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 ii u mediately 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: theMiiller- 

 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 



Fio. 244. Chick extracted from egg during seventh day S0 .^^ bot]l ''" "'' ! (> 

 towing oprentam (<p), and pulmonary eavi- 



ties are completely 



ensheathed by muscle. The splitting apart of the t\v<> veaselfl is 

 inaugurated by the appearance <r a Longitudinal incision along th i 

 line of attachment of the septum. 



regards the further progress of development, the I' 



appm\ imate i imef may '" mentioned. 



Altout tlie ninth lay the (.esoplia^us gradually heeomes patent 

 again. On the tenth day tin- artri-ial aivhrs have praetieally assumed 

 the definitive ( nnd it ion and the m. -tap .dial skeleton is ossified. 



