LATER DEVELOPMENT OF CHICK AND RABBIT 421 



flattened sac, which by the twelfth day has spread almost entirely 

 around the blastoderm. It fuses with the chorion to form a com- 

 bined structure, the allanto-chorion. This is a very important 

 organ in the physiological life of the actively growing embryo. The 

 circulatory system of this organ is composed of two large allantoic 

 arteries and one large allantoic vein connected by a complicated 

 plexus of anastomosing vessels. The allanto-chorion receives the 

 nitrogenous excretion of the embryo, is the organ of respiration, and 

 also greatly assists in the absorption of the albuminous white of 

 the egg. 



Gut and Related Structures. 



In the formation of the gut three regions can be distinguished : 

 the first is the fore gut, formed, as we have seen, by the pulling in of 

 the entoderm during the formation of the head fold ; the second in 

 order of formation is the hind gut, resulting from the formation of 

 the tail fold ; and the third is the mid gut, which joins the two 

 preceding portions, and is completed by the closing in of the 

 splanchnopleure all along, save at the yolk sac opening. I he first 

 two are the most important. 



As the head fold forms, the ectoderm on its underside bends up, 

 touches and fuses with the entoderm of the front end of the fore gut 

 to form the oral plate. This is at first superficial, but, with the 

 flexure of the front end of the embryo, it becomes overhung by a 

 ridge, and shortly after the mesoderm at its sides thickens to form 

 two ridges, the mandibular arches, and so the plate comes to lie at 

 the bottom of a shallow depression, the oral fossa or stomodoeum. 

 At the beginning of the third day the oral plate ruptures, putting the 

 lumen of the fore gut in communication with the stomodceum. 

 Even before this time the dorsal stomodceal wall gives off an out- 

 growth that grows in under the floor of the fore-brain, and later 

 its terminal portion becomes detached and forms the glandular 

 portion of the hypophysis cerebri. The formation of the jaws 

 greatly enlarges the stomodoeum, which gives rise to the buccal 

 cavity of the adult, and hence the latter is lined by ectoderm. 



From the fore gut arise the pharynx with the branchial pouches, 

 lungs, oesophagus, stomach, duodenum, liver and pancreas. The 

 front end of the gut is the pharyngeal region, and along four trans- 

 verse lines on each side it bulges to form a series of grooves, the 

 visceral or branchial pouches. The first of these to appear is the 

 hyomandibular pouch, which is noticeable at 38 hours (14-16 

 somites) as a groove on each side just in front of the level of the 

 auditory pits. Its deeper portion reaches and fuses with the ecto- 

 derm to form a branchial membrane. The second pouch appears at 



