478 DEVELOPMENT OF THE CHICK. 



4. The Third Day. The Amnion. 



As the embryo increases rapidly in weight, especially at 

 its head-end, it tends to sink down into the soft yolk. This 

 tendency is probably the reason why certain folds of the 

 somatopleure (head-fold, lateral folds, tail-fold) gradually 

 enclose and surround the body. These folds constitute the 

 ' amnion ' or caul (figs. 86, 90, AN), which looks like a sort of 

 veil. The space between the two layers of the amnion is part 

 of the ccelom. 



Fig. 89. Transverse section across the head of a Chick Embryo at the 

 twent\ -fourth hour of incubation, passing through the region of the 

 mid-brain, x 100. 



B, cavity of the mid-brain. CH, notochord, not yet separated from the 

 hypoblasts wall of the pharynx. E, epiblast. H, hypoblast. NA, neural 

 ridge. RT, commencing heart. TP, pharynx. 



A. External Features. 



The great size of the head is a marked feature of this and 

 succeeding stages, and the flexure both of the fore-brain on 

 the hind-brain and of the head upon the trunk are equally 

 noticeable. The head is also twisted over to the left side so 

 that the body is no longer symmetrical. The trunk has 

 elongated and the number of body-sigments (as indicated by 

 the somites) has imreased. There is still no trace of wing 

 or leg.' The hyoidean and first three branchial clefts become 



