Embryology. 17 



coalesce and then the trench becomes a tube. This is called the medul- 

 lary tube and it afterwards becomes the seat of the spinal cord. It now 

 becomes apparent that the future animal lies on its belly upon the cen- 

 tral part of the egg. Its back is the outside or top of the germ shield, 

 the wide end of the germ shield is to be the head and the narrow end 

 the tail. Just under the medullary tube, and running parallel with it 

 and lengthwise of the shield, another long cylindrical body becomes de- 

 tached from the skin fibrous laj^er or flesh stratum. This cylinder is 

 the notochord or the rudimentary backbone. It occupies the position 

 afterwards occupied by the transverse processes of the backbone, and 

 the oblique processes, when they are formed, reach around and enclose 

 the medullary tube which then becomes the spinal canal. All around 

 the edges of the germ shield the outside skin stratum together with the 

 flesh stratum, now begin to form a ridge by a swelling fold of these two 

 layers. This fold or doubling of these skins continues to rise and 

 reach over the shield from all sides, but more rapidly over the head and 

 tail than the sides; until finally the folds touch each other and coalesce 

 over the back of the shield. (See Fig. 22.} 



The edges of doubled membrane, where they meet and touch each 

 other, form partitions which are soon absorbed in such a way as to 

 make the space between the upper aud under folds of the skin layer 

 continuous across the shield, and the shield is left in a cavity canopied 

 by two continuous and independent sheets of the original skin layer. 

 This operation, by doubling or folding the skin stratum, causes the part 

 of it on the inside of the cavity to face toward the shield and leaves the 

 rest facing as before in the contrary direction toward the prochorion or 

 original outer shell of the egg. This outward-facing part of the skin, 

 layer now moves away from the other part and the space between them 

 is filled with fluid. The outside portion is gradually pressed against 

 the prochorion with which it appears to fuse and so form the permanent 

 chorion. The other inner part of the skin-fold that roofed over the 

 cavity above the germ-shield becomes the amnion sac. It becomes filled 

 with fluid and gradually enlarges and distends itself until later it en- 

 tirely surrounds the embryo. 



While this process is going on above the shield, the skin stratum just 

 below the edge of the shield, is becoming constricted or drawn in all 

 around and especially on the sides of the shield, a process which has a 

 tendency to make another tube or cavity on the underside of the shield 

 below the notochord and at the same time to separate the shield, or em- 

 bryo as we ma} r now call it, together with its covering, the amnion sac, 

 from the lower and larger part of the original egg. This makes a sac 

 of this larger part which is called the yelk sac. The cavity thus formed 

 from which the principal part of the yelk sac is thus partly separated, is 



