894 



EMBRYOLOGY. 



INTERNAL LAYER. 



a. Intrafo2tal parts. The mucous layer of the blastoderm forms the 

 epithelium of the intestines, the glands annexed to them, the respiratory 

 apparatus, the kidneys, and the bladder. These organs are developed at the 

 same time as the extra foetal portions, and will be referred to presently. 



b. Extrafatal parts. In incurving on itself, the embryo incloses in its 

 cavity a portion of the internal blastodermic layer. The union between the 

 free and the imprisoned portions is extensive ; but before long it is only 

 represented by a narrow canal that occupies the inferior umbilical ring. 

 The mucous layer is then divided into two distinct portions : the intestinal 

 groove, or intrafcetal portion; and the umbilical vesicle (vesicula alba), or 

 extrafcetal portion ; they communicate by the omphalo-mesenteric or vitelline 



Fig. 420. 



Fig. 421. 



PLAN OF EARLY UTERINE OVUM. 



Within the external ring, or zona pellucida, 

 are : the serous lamina, a ; the yolk, 6 ; and 

 the incipient embryo, c. 



DIAGRAM OF OVUM AT THE FORMATION 

 OF THE AMNION. 



a, Chorion ; 6, Yolk-sac ; c, Embryo ; d and e, 

 Folds of the serous layer rising up to form 

 the amnion. 



The umbilical vesicle is filled with a granular fluid, which is conveyed, 

 for the nutrition of the foetus, by the omphalo-mesenteric vessels. When 

 this alimentary reserve is nearly expended, the allantois appears. 



The allantois commences by a small enlargement, which is thrown out by 

 the intestinal furrow on the inferior abdominal wall. This bud becomes 

 elongated, and gradually enlarges by drawing the umbilical vessels to its 

 border : appearing as a small vesicle that at length extends through the 

 umbilicus, and spreads itself over the inner face of the chorion, between it 

 and the amnion. 



This new organ is divided into two sacs by a neck the uracJius at 

 the umbilical ring ; the internal is the smallest and forms the bladder, and 

 the external, the most voluminous, constitutes the allantois proper. 



