FORMATION OF THE LAYERS OF THE BLASTODERM. 659 



the following manner. At the front of the area pellucida a fold, 

 or dipping in of the blastoderm takes place ; this consists of a 

 projecting part or fold above and a groove below, and consti- 

 tutes what is known as the cephalic or head fold. The upper 

 projecting portion of the fold tends to grow forwards, whilst the 

 groove grows gradually backwards. Later on, another fold 

 appears at the posterior part of the area pellucida ; this is the 

 tail fold. At the sides of the area pellucida folds appear, which 

 tend to grow downwards and inwards so as to reach the under 

 surface of the blastoderm and unite with the head and tail folds. 



FIG. 252. 



NC. 



F.So. 



Diagrammatic longitudinal section through the axis of an embryo chick. 

 (Foster and Balfour.) N.C. Neural canal. Ch. Notochord. D. Foregut. 

 F.So. Somatopleure. F.Sp. Splanchnopleure. Sp. Splanchnopleure form- 

 ing the lower wall of the foregut. Ht. Heart, pp. Pleuroperitoneal cavity. 

 Am. Amniotic fold. A. Epiblast. B. Mesoblast. C. Hypoblast. 



By the approximation of all these folds a canal is formed the 

 embryonal sac which is closed above by the main portion of the 

 area pellucida, in front by the head fold, behind by the tail fold, 

 at the sides by the lateral folds, whilst below it is open to the 

 vitellus. This canal ultimately becomes subdivided into an inner 

 tube, the alimentary tract, and an outer one, which forms the 

 body walls, the final place of union of the folds being marked by 

 the umbilicus. It must be clearly understood that these primary 

 folds which form the embryo include in their layers the epiblast, 

 the whole thickness of the mesoblast, and the hypoblast, whereas 

 the folds giving rise to the membranes do not comprehend all 

 these layers. 



