552 EMBRYOLOGY OF THE LOWER VERTEBRATES < n. 



am. 



there pass out the stalks of the yolk-sac and the allantois (Fig. 246, B) 

 each conspicuous owing to its large blood-vessels. The peripheral 

 distribution of the vitelline and allantoic vessels shows a characteristic 

 difference (Fig. 242) the vitelline network (vascular area) terminating, 

 in the now greatly reduced terminal sinus at a considerable distance 

 from the distal pole of the yolk-sac while on the other hand the allan- 

 toic network is most richly 

 developed on the distal 

 side of the allantois (p. 

 474). 



The body of the em- 

 bryo now for the first time 

 begins to show indications 

 of bird -like form, and 

 faint traces of digits and 

 of feather-rudiments may 

 become apparent about 

 the end of the day. 



In the eye the rudi- 

 ment of the pecten, which 

 first became recognizable 

 during the fourth day, is 

 now conspicuous as an 

 ingrowth of mesenchyme 

 through the choroidal 

 fissure, bounded on each 

 face by the inflected lips 

 of the fissure. 



The tongue begins to 

 project and the thyroid 

 becomes constricted off 

 from the pharynx. The 

 oesophagus towards the 

 end of the day loses its 

 cavity; the dilatation of 

 the gizzard becomes evi- 

 dent ; the intestine begins 

 to grow actively in length (Fig. 246, B). The three pancreatic 

 rudiments become continuous with one another. 



The muscles of the body begin to exhibit contractility, tin- trunk 

 occasionally showing twitches of vent ral ilexure. Tin- ureter develops 

 outgrowths to form tin- primary collecting tubes of the melane] lin>s 

 about, tin- I't^innmi! of tin- ,-i\th or the end of the fifth day and the 

 leimiual part of tin- dud of the opisthoiiephros may heeomr incorpor- 

 ated in the cloaca so as to ^ive the ureter its [ndependeirl oju-niu^. 

 About this lime tin; first indications of sexual different iation heeoine 

 recogni/ahl-. tin- ;jvniial strands lu-^i lining to show siuns of degenera- 

 tion in th- female 



a.u 



alb. 



Fi<;. 242. Common Fowl. View of contents of the 

 egg-shell extracted at the end of the sixth day of 

 incubation. The serous membrane has been removed 

 as to allow the allantois to be displaced slightly 

 in order to give a clearer view of the body of the 

 embryo contained within its amnion. 



a.r, edge of vascular area; ulb, remains of albumrii ; '///', 

 outer wall of all.-mtois ; nil", inner wall of allantois; am, 

 arnnion ; *, portion of vascular area lying, in the natural 

 position, benwath the head of the rmbryo and free from blood- 

 vessels. 



