CHAP.XLI.] BRANCHIAL FISSURES AND ARCHES. 589 



Von Baer. Reichert, however, considers that it is formed in a 

 somewhat different manner to that above described."* 



In mammalia, the formation of the amnion occurs at a very early 

 period, usually within twenty- four hours after the first appearance 

 of the primitive trace. 



About the twentieth day the ovum is about ten, and the embryo 

 is between three and four lines in length. The villi of the chorion 

 are beginning to make their appearance, and gradually extend 

 themselves round the ovum. That portion, however, which is im- 

 mediately beneath the embryo is the last part of the surface upon 

 which villi appear, while the two ends of the ovum always remain 

 smooth and uncovered with villi. 



By this time, with the exception of their posterior portion, the 

 laminae dorsales have coalesced, and the vertebral plates have 

 increased in number. The position of the eye and ear can be 

 determined, and the anterior surface of the body is being gradually 

 closed in by the convergence of the ventral lamince which spring 

 from the under part of the dorsal laminae. At length the body of 

 the embryo appears entirely pinched off as it were, and is only 

 connected with the yolk sac or umbilical vesicle by a narrow 

 constricted portion, the omphalo-enteric duct. 



.Branchial Fissures and Arches. At this period, three branchial 

 fissures, and three visceral arches may be detected. 



The term branchial arch is a bad one, since it conveys the idea 

 that, at a certain period, branchiae are de- 

 veloped in the higher vertebrata, which is not 

 the case. The only structures representing the 

 branchial vessels of fishes are branches which 

 unite to form the descending aorta. In fishes, 

 the arches become permanent, and are sur- 

 mounted by branchial laminae. They are five in 

 number ; at a very early period there are six, but 

 one is not further developed. In the amphibia 

 a similar process takes place, but here the 

 branchiae waste away when the animal assumes 

 its adult form. Usually only two, very rarely 



r> i .. .., i !_ j Visceral or branchial 



four, arches remain in reptiles ; and in birds, arches of an embryo dog, 



-i. -, , . . , after Bischoff. a. brain; 



mammalia, and man, only one is persistent. &. eyes? c. a. right and 

 Heart and large A rteries. About this time also a nd I2?vwitricies ; g. aorta 



ji v -i i / i mi i A. divi-ling into aortic arches. 



the heart and large vessels are formed. The heart i, 2 , 3, 4, corresponding 



appears first in the form of a narrow elongated 



Miiiier's Physiology, translated by Dr. Baly, p. 1552. 



