x FOWL— THIED DAY 545 



general sketch of the chief advances in development which take 

 place during this day. 



At the commencement of the day the body of the embryo lay 

 flat along the surface of the yolk: only at its head end was it 

 clearly demarcated from the surrounding blastoderm and this head 

 region owing to the commencing ventral curvature was beginning 

 to lean over on to its left side. During the course of the third day 

 the tucking in of the blastoderm under the definitive body proceeds 

 apace so that the body becomes more and more completely demarcated 

 from the part of the blastoderm forming the yolk-sac wall, and the 

 yolk-stalk becomes correspondingly narrowed. The preponderance 

 of growth activity on the dorsal side which leads to the ventral 

 curvature is during the early hours of the day especially marked in 

 the region of the mesencephalon but as the day goes on becomes 

 very pronounced about the level of the heart and still later in the 

 tail region. Thus the axis of the body develops strong ventral 

 curvature especially marked at three different levels — mesencephalic, 

 cardiac and caudal. Along with. this increasing curvature the whole 

 body of the embryo comes to lie over on its left side so that the 

 observer looking down upon the egg from above sees the body of the 

 embryo in profile from its right side. 



During the day the embryo becomes ensheathed in the amnion 

 in the manner already described. The vitelline network of blood- 

 vessels attains to its highest development, forming as it does the 

 organ for respiration as well as for absorption of the food and its 

 transport into the body of the embryo. Correlated with the lying 

 of the embryonic body over on its left side the paired venous 

 channels which convey the blood from the vitelline network into 

 the heart gradually lose their symmetry, those of the right side 

 dwindling in size while their fellows show a corresponding increase. 



In the brain the main regions become established : the roof of 

 the thalamencephalon and medulla oblongata assume their thi^i 

 membranous character while the hemispheres bulge out in front 

 of the thalamencephalon. The central canal of the spinal cord 

 becomes reduced to a vertical slit by the thickening of the side walls. 

 The olfactory rudiment makes its appearance : the auditory rudiment 

 becomes converted into the closed pear-shaped otocyst, still however 

 connected with the ectoderm by a solid strand of cells. In the eye 

 the lens thickening has become involuted and converted into a 

 closed vesicle with its inner wall markedly thickened. The optic 

 cup has been completely formed and the retinal layer differentiated 

 from the thin and degenerate pigment layer. In the latter the first 

 deposition of pigment takes place during the later hours of the day. 



The definitive alimentary canal is still open towards the yolk-sac 

 over about half its extent but in addition to the foregut there 

 becomes folded off during the course of the third day a considerable 

 extent of hind-gut, the ventral wall of which commences to bulge 

 out to form the rudiment of the allantois towards the close of the 



VOL. II 2 N 



