DEVELOPMENT OF THE AMNION. 657 



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

 projecting part or fold above and a groove below, and constitutes 

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

 jecting portion of the fold tends to grow forward, whilst the 

 groove grows gradually backward. Later on, another fold ap- 

 pears 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 downward and inward so as to reach the 

 under surface of the blastoderm and unite with the head and 

 tail folds. 



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

 embryonal sack 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. 



FORMATION OF THE MEMBRANES. 



(1) The Amnion. The mesoblast around the embryo becomes 

 thickened, and is split into two distinct layers ; this cleavage is at 

 first confined to the neighborhood of the embryo, but gradually 

 spreads over the whole blastoderm. 



The upper of these two layers of the blastoderm receives the 

 name of the somatopleure, and is engaged in the formation of the 

 body walls of the embryo and the amnion. The lower one is 

 called the splanchnopleure, and forms the walls of the alimentary 

 canal, the allantois, and the yelk sack. The space intervening 

 between these layers is called the pleuroperitoneal cavity. At a 

 point in front of the cephalic fold, an upward projection of soma- 

 topleure takes place, conveying with it the overlying epiblast. 

 Along the sides of the embryo and behind the caudal fold pro- 



