2IO 



EMBRYOLOGY 



and knee are formed, but all the limbs are still exceedingly alike in 

 shape. Most of the skeletal parts make their appearance as car- 

 tilages ; especially the cartilaginous cranium, the visceral arches 

 like the jaws and hyoids, the vertebrae with the ribs, and the frame- 

 work of the limbs. The changes affecting the vascular system have 

 already been mentioned. 



Changes from the 6th day onwards. — 



The sixth day marks a new epoch in the development of the 

 chick, for distinctly avine characters then first make their appear- 

 ance, the embryo of a Bird being until this period strikingly like 

 that of any other amniotic vertebrate, for instance, that of a Mammal 

 or still more like that of a four-footed Reptile. The avine specializa- 

 tion begins of course very gradually. But on the sixth day for the 



Am. 



Embryo of Fowl on the Sixth Day. Umbilical Region of the Same. 



Natural size and. position. Magnified. 



Am. Amnion ; Al. Allantois ; G, Gut ; T, Tail ; IF, Wing ; y.s. Tolk-sac. 



first time become visible the main features of the characteristically 

 avine wing and foot ; the crop and the intestinal cseca make their 

 appearance, the stomach is diff"erentiated into a proventriculus and 

 a gizzard ; the nose begins to develop into a beak ; and the com- 

 mencing bones of the skull arrange themselves after an a"\ane 

 type. 



During the seventh day the amniotic cavity has assumed con- 

 siderable dimensions, the fluid increasing Anthin it ; obvious move- 

 ments begin to appear in the amnion itself as slow vermicular 

 contractions which creep rhythmically over it. The amnion in fact 

 begins to pulsate slowly and rhythmically, and by its pulsations the 

 embryo is rocked to and fro in the egg. The allantois, which at a 

 later period shews similar movements, has spread out rapidly in 

 all directions and is filled A^ath fluid. The vascular area loses 

 its terminal sinus ; both vitelline arteries and veins now pass to 

 and from the body of the chick as single trunks, assuming more 



