CHAP. X.] 



THE DEVELOPMENT OF THE CAT. 



325 



Meantime, during the development of the amnion, another structure, 

 of great importance to the embryo, is being formed. This is the 

 ALLAXTOis (Fig. 148, 3 & 4, al), which buds forth from the mesoblast 

 beneath the hinder portion of the incipient abdomen. It soon attains 

 a large size, and becomes supplied with blood-vessels for a purpose 

 to be presently described. 



While the amnion and allantois are thus being formed, the side 

 walls of the embryo are closing in by the median junction ventrally 

 of the somatopleure of one side with that of the other side, and 

 the splanchnopleure of one side with that of the other side, thus 

 enclosing — between the inner layer of conjoined splanchnopleures — 

 a space which is the future alimentary canal (Fig. 147, C and D). 

 This is at first closed in front and behind, and everywhere, except 

 at one ventral point, where it remains open, and in connexion with 

 the remains of the yelk, which, with its enveloping membrane, is 

 termed the umbilical vesicle (Fig. 147, C, ti). 



On each side of the medullary groove and notochord a series of 

 quadrate thickenings appear, termed protovertebrce (Fig. 146, pr), 

 which are the first signs not only of the vertebrae, but also of the 

 muscles and nerves connected with them, to all of which they 

 give rise. Inflections of the epiblast about the incipient head, lay 

 the foundations of the organs of special sense, while an epiblastic 

 inflection at each end of the alimentary tube forms respectively the 

 buccal and anal chambers. In the mesoderm, immediately beneath 

 the anterior end of the chorda, the blood and blood-vessels become 

 developed, and one vessel, tubular and rythmically contractile, lays 

 the foundation of the heart. Blood-vessels at first go to and fro 

 between the umbilical vesicle and the heart to absorb nutriment 

 from the yelk, but subsequently these are outstripped in development, 

 and so replaced by other vessels which go to and fro between the 

 allantois and the heart. Thus the great blood-vessels have at first a 

 very different course and arrangement from that which they ultimately 

 attain. About the alimentary canal, the liver arises, while the trachea 



More advanced embryo, showing tlie proto- 

 vertebite, \iseeral arches, and rudimentary 

 limbs. The allantois has grown out to the 

 inside of the chorion, and conveyed vessels 

 to it. The villi are more complex. The 

 unrbilical vesicle is small, and connected 

 with the embryo by a long duct. In the 

 embryo cat, however, this vesicle is spindle- 

 shaped, and of considerable size, and the 

 duct is short. 



e. Embryo. 



a. Epiblast. 



m. Mesoblast 



i. Hyjioblast. 



am. Amnion. 



Is. Head-fold. 



ss. Tail-fold. 



ah. Cavity of amnion. 



as. Amnionic sheath of the umbilical 

 cord. 



kh (Fig. 1). Hollow of the germ-vesicle, 

 which becomes (t/s) that of the yelk- 

 sac. 



il{j. Duct leading from the alimentary 



canal to the yelk-sac. 

 ('/(Fig. 3). Lining of umbilical vesicle or 



yelk-sac. 

 (hi. Dorsal wall of intestine. 



d. 



or pnmi- 



Cli: 



Yelk or vitelline membrane, 



tiye chorion. 



Permanent chorion, with which not 



only the onter layer of the anmionic 



folds (s/(), but also the allantois, has 



combined. 

 ol. Allantois. 

 )■. Space, filled with fluid, between the 



amnion and chorion. 

 hh. Kegion of the heart and pericardial 



cavity. 

 vl. Wall of the thorax in the region of the 



heart. 

 (V (Figs. 1, 2, and ?,); sz (Fig. 4) and 



ch z (Fig. 5). Villi of chorion. 

 iU (Fig. 3). Outer layer of amniotic fold, 



which, in Fig. 4, has coalesced with 



the inner wall of the primitive 

 chorion. 



