464 ZOOLOGY FOR MEDICAL STUDENTS chap. 



lateral portions of the fold appear later and develop more slowly. When 

 the amniotic opening has completely disappeared the result is that the 

 body of the embryo is covered in by two membranous roofs. Of these 

 the inner, formed of the inner layer of the original amniotic fold, is the 

 amnion in the strict sense (" true amnion," Fig. 195, D, am), while the 

 outer formed from the outer layer of the original fold is known as the 

 false amnion (J. a). The latter is simply continuous without a break with 

 the rest of the somatopleure which extends outwards towards the limit 

 of the blastoderm. 



The cavity of the amnion, like other spaces in the developing egg, is 

 full of secreted watery fluid and it forms a little tank in which in later 

 stages the delicate body of the embryo floats, protected from injury by 

 the sudden jars to which under terrestrial conditions eggs are exposed. 



As regards the evolutionary origin of the amnion nothing is definitely 

 known but on the whole it seems most probable that it arose through 

 the body of the embryo, increasing in size within the confined space 

 limited by the egg envelopes, being forced down in amongst the yolk, 

 the surface of the blastoderm bulging up all round (Fig. 195, A). The 

 network of blood-vessels in the blastoderm naturally performs a respiratory 

 function, gas-exchange taking place between the blood in them and the 

 external atmosphere. Consequently there would be a tendency towards 

 increase in area of the blastoderm, and the bulging inwards over the 

 surface of the embryo would tend to increase to the utmost limit until 

 at last the amniotic opening would be obliterated (Fig. 195, B). With 

 the splitting of the mesoderm to form the coelome the splanchnopleure 

 would be withdrawn from the amniotic fold and the latter would now 

 be a fold merely of somatopleure (Fig. 195, C). 



Towards the end of the third, or early in the fourth, day the blood 

 system assumes very interesting conditions. The heart is still in the 

 form of a tube and its contraction, beautifully visible in the egg opened 

 under warm salt solution, takes the form of waves which pass from 

 behind forwards along its length. The tube no longer bulges simply 

 towards the right but shows a double curvature, something like 

 an S, the dorsal limb of which is destined to give rise to the atrium 

 (Fig. 196, A, at) and the ventral to the ventricle (Fig. 196, A, V)?- The 

 latter is continued forwards as a short ventral aorta which gives off on 

 each side three or four aortic arches (Fig. 196, A, I-III) between which 

 are gill clefts. Dorsally the aortic arches of each side open into an aortic 

 root and these are continued back into the unpaired dorsal aorta. In 

 front the aortic root passes forwards into the head as the dorsal carotid 

 1 Compare with the Elasmobranch heart as shown in Fig. 134, p. 321. 



