BIRTH OF THE CHICK. 333 



the testes delicate vasa efferentia are developed, which, after 

 passing through the Wolffian bodies, unite into a filiform vas 

 deferens, which in its turn is evolved out of, or, more cor- 

 rectly, into the excretory duct of the Wolffian body. The vitel- 

 lary sac shrinks more and more, its contents diminishing in 

 quantity, and becoming still more consistent. It is drawn into 

 deep sacculated compartments by the main trunks of the um- 

 bilical vessels ; the albumen and amniotic fluid are lessening 

 continually in quantity. The tegumentary umbilicus is still 

 freely open at the beginning of the last week ; and with the 

 advancing growth of the intestinal canal, a greater number of 

 convolutions of the bowel pass out of the abdominal cavity ; 

 on the nineteenth day the prolapsed intestine returns in some 

 degree into the abdomen again, and draws the yolk, with 

 which it is still in uninterrupted connexion by means of the 

 very considerable vitellary duct, along with it into the belly, 

 upon which the mucous and vascular layers of the vitellary 

 sac follow, whilst the serous layer increases, becomes thicker, 

 and detaches itself from both the other layers. The whole 

 vitellary sac is not thus taken up into the abdomen, only a 

 part of it enters, and this expands in the cavity, whilst the 

 part that is excluded is cut off by the contracting umbilical 

 ring. The vitellary duct is of considerable width, and arises 

 funnel-shaped from the intestine ; long after birth there is still 

 a little diverticulum of the jejunum to be discovered in its for- 

 mer situation ; nay, in some birds this diverticulum continues 

 through life as a normal feature in their structure. The com- 

 munication with the vitellus is at length obliterated, becoming 

 a mere thread, on which a yellow knot, the last remains of the 

 yolk, may not unfrequently be observed. 



BIRTH OF THE CHICK. 



[ 498. Two days before its exclusion, the chick may occa- 

 sionally be heard chirping feebly within the shell, for the cho- 

 rion (the allantois) is readily torn by the point of the beak, 

 which then comes into contact with the air contained in the 

 air-chamber ; along with the imperfect respiration that now 

 goes on, the circulation through the umbilical vessels proceeds 

 unimpeded. The violent motions of the chick occasion cracks 

 in the shell ; the beak assists, and holes are produced. The 

 bill, so soft in all other parts, is furnished at this period with 



