THE ORGANS OP THE INNER GERM-LAYER. 



283 



embryo, because with their appearance the head- and neck-regions 

 become distinguishable. 



A. The Development of the Mouth. 



In all vertebrated animals the epidermis forms on the under side- 

 of the rudimentary head, which at first has the appearance of a 

 rounded knob, a small shallow pit (Plate I., fig. 11, and fig. 151),. 

 which meets the blind end of the 

 fore gut (kd). In the region of 

 this pit the middle germ-layer 

 is from the beginning absent 

 (KEIBEL, CARIUS). Outer and 

 inner germ-layers meet to form 

 a thin membrane (fig. 151 rh), 

 which separates oral sinus or 

 oral pit [stomodaeum] and fore 

 gut, and which has been de- 

 scribed since the time of REMAK 

 as pharyngealmembrane (Eachen- 

 haut). By its rupture and the 

 degeneration of the shreds of it 

 known as the primitive palatal 

 velum communication with the 

 outside is established (Plate I., 

 figs. 4 and 7 m). 



Fig. 152. Human embryo (Lg of His) 2'15 mm. 

 long, neck measurement* Drawing from 

 a reconstruction, after His (" Menschliche 

 Embryonen"). Magnified 40 diameters. 



Mb, Oral pit (or sinus) ; Ab, aortic bulbus ; 

 Vm, middle part of the ventricle of heart , 

 PC, vena cava superior or ductus Cuvieri ' r 

 Sr, sinus reuniens ; Vu, vena umbilicalis ; 

 VI, left part of the ventricle ; Ho, auricle of 

 heart ; D, diaphragm ; V.om, vena omphalo- 

 mesenterica ; Lb, solid fundament of the 

 liver ; Lbg, hepatic duct. 



In the case of the Chick the oral 

 pit is observable on the second day 

 of incubation, the front end of the 

 embryonic fundament having a short 

 time previously elevated itself as a 

 cephalic knob above the extra-em- 

 bryonic part of the germ-layers. The rupture of the pharyngeal membrane 

 takes place on the fourth day. In the case of an embryo Rabbit of nine days 

 the pharyngeal membrane is not yet ruptured. His has studied in detail this 

 early stage in Man on his embryo " Lg," the age of which he estimates at twelve 

 days. 



In all amniotic Vertebrates the entrance to the oral pit (fig. 152 

 Mb) presents a very uniform condition and appears as a large five- 



* [It will be seen by an inspection of figure 158 that the longest straight line 

 which can be drawn through the embryo connects the neck- and rump-regions. 

 It is this distance which is designated as the neck, or neck-rump, measure- 

 ment.] 



