THE AMNION. 955 



unite along the middle line, to form a canal, containing the rudiments 

 of the future brain and spinal cord ; the anterior part or cephalic end 

 of this canal becomes more expanded than the rest, whilst the poste- 

 rior part tapers to a point. In this way, the so-called neural cavity 

 (p. 114) of the future animal, is formed above the chorda dorsalis, 

 traces of which are, for a time, found passing through the bodies of 

 the growing vertebrae. Soon, from the vertebral plates, where these 

 turn upwards, the external and middle layers extend sideways, consti- 

 tuting the so-called lateral plates, which, growing downwards, and 

 bending inwards, form the walls of the abdomen, and inclose a cavity 

 which is placed beneath the chorda dorsalis, immediately in contact with 

 the yolk ; this ultimately constitutes the hcemal or thoracico-abdominal 

 cavity of the future animal ; it soon contains the heart and great 

 bloodvessels, and the rudiments of the alimentary canal, which is 

 completed by a corresponding folding-in of the internal blastodermic 

 layer, which lies immediately upon the yolk. 



The embryo, by the bending in of its sides, next appears to be raised 

 from the yolk, and partially shut off from it, by a sort of constric- 

 tion which takes place, first beneath the head and the caudal ex- 

 tremity, and afterwards at each side. Ultimately, this constriction 

 shuts off the body of the embryo from the yolk-sac, which then com- 

 municates only by a narrow passage, the ductus vitelli, or vitelline 

 duct, with the central space in the interior of the embryo, now the 

 rudimentary alimentary canal, lying in the haemal cavity. The yolk- 

 sac thus cut off, shrinks, and forms the umbilical vesicle. The head of 

 the embryo, now free, bends down towards the yolk, and forms the 

 cephalic flexure. At the same time, a delicate transparent mem- 

 branous fold, derived from the external germinal layer of the blasto- 

 derm, rises, like a hood, over that part of the embryo ; a similar, but 

 smaller fold, rises over the free caudal extremity ; and on each side, 

 where the lateral plates bend in to form the constriction just described, 

 corresponding folds arise. These folds are double ; they grow, and at 

 length meet over the back of the embryo, and coalesce so as to form, 

 by their innermost layer, a complete, but delicate, closed sac, called 

 the amnion, which, in the chick, is perfected as early as the third day 

 of incubation. This is at first close to the embryo, but it soon expands, 

 and carries with it the outer layer of the same folds, which afterwards 

 reaches the shell membrane in the Bird, but in the Mammalian ovum 

 becomes attached to the inner surface of the chorion, and so forms the 

 false amnion. The sac of the amnion surrounds the vitelline duct, in 

 a sort of sheath, and thence becomes continuous with the skin cover- 

 ing the body of the embryo (see Fig. 121). 



The amnion is thin, transparent, and non-vascular. It consists, at 

 first, of a structureless basement-membrane, lined with a delicate squa- 

 mous epithelium ; afterwards, it contains fusiform cells and a fine areo- 

 lar tissue, and, in Birds, even non-striated muscular fibres. Its fluid 

 contents, the liquor amnii, usually alkaline, consist of water, having 

 in solution from 1 to 3 per cent, of solid matter : this is composed of 

 a little albumen, traces of urea and uric acid, allantoin, and other ex- 

 tractives, salts, such as lactate of soda, and sulphate and phosphate of 



