58 



THE EARLY DEVELOPMENT OF MAMMALS. 



The Oral and Anal Plates. 



These two structures resemble one another. Each occupies a small area and is 

 formed by the intimate union of the entoderm with the ectoderm. When the union 

 is first formed the two layers are distinct, but they soon fuse, so that no boundary 

 can be recognized between them. Ultimately both plates break down, their cells 



Am. 



Cho. 



Bs. 



- U.A. 



Yk.s. 



fo. 



FIG. 25. WAX RECONSTRUCTION OF DANDY'S HUMAN EMBRYO WITH SEVEN SEGMENTS BY FREDERICK T. LEWIS. 



All, Allantois. Am, Amnion. Bs, Body stalk. Cho, Chorion. fo, Fovea cardiaca. H.g, Hind-gut. Ht, Heart. 

 Kn, Hensen's knot. Som, Somatopleure enclosing the pericardial cavity. U.A, Umbilical artery. U.V, 

 Umbilical vein. Vd, Fore-gut, ve, Blood-vessel. Yk.s, Wall of yolk-sac. X 40 diams. 



disappearing, and they are replaced by openings, that of the oral plate forming the 

 opening between the mouth-cavity and the pharynx, that of the anal plate forming 

 the primitive anal opening. The anal plate, before it breaks down, makes a con- 

 siderable growth, forming an epithelial mass which plays an important part in the 

 anatomical modeling of the region. The oral plate disappears very early; the anal 

 plate much later. 



