A.VD THE POETAL MEMBRANES. IN THE CHICK. 3^5 



tion. The yoJk-sac becomes then enclosed by the abdominal wall and 

 the entire s])ac(' of the egg becomes filled by the amnion which is 

 occupied by a fuJly orown embryo. 



Contrai-y t(j the observations of some writers, the Jeft aJhmtoir 

 artery is found to [)ersist. th(ju;^-h faintly, till the last day (Figs. 27, 

 2S, o2, and o3), while the allantoic vein and the right allantoic artery 

 are of great functional impoi-tance throughout the allantoic life. The 

 inner allantoic limb is always sup|)lied by much fewer blood vessels 

 than the outer limb. 



Finally, the sero-amniotic connection is quite without relaticju 

 to the emergence of the embryo, which pushes out its beak into the 

 air chamber through the point of the membrane far distant from tlie 

 connection. 



10. Comparison betiveen llw Sero-Amniodc Connection 

 of the Chick and lliat of Chriniia. 



'I'lie sero-amiiiotic connection of the chick, as des('ril)ed above, and 

 that of Cliclcjnia, as studied by Mitsukuri in Clrnnni/s und Trionyx, mav 

 be compared as follows : — 



Tlie essential points in irliich fJiei/ atjrce. — a). In the chick, there 

 is found at the p(Jsterior edge of the amniotic fold a constant deltoid 

 area free from the mesoblast. The same sti-ucture is observed in 

 Chelonia in comparatively later stages of the progress of the fold 

 backwards. 



/)). As long as the amniotic fold grows p(3steriorlv the sei'o- 

 amniotic connection or the remnant of the epiblastic deltoid ai-en is 

 equally prolonged posteriorly until it becomes a long string. 



c) The sero-amniotic connection is more or less widened on 

 both sides of the epiblastic bridge by the entrance of the netted meso- 

 blastic tissue. 



