llli: ORGANS OF TIN: [NNEH GERM-LAYER 





embryo, because with their appearance the head- and neck-re^ 

 become distinguishable. 



A. Th>' l)>'r>'l<'i>nt',<t <>/' (/<> Mniith. 



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. I-~>1>, 



whk-h meets the blind end of the 



fore gut (kd). In the region of 



this pit the middle germ-layer 



is from the beginning absent 



iKKiiiEL, CARIUS). Outer and 



inner germ-layers meet to form 



a thin membrane (fig. 151 rh\ 



which separates oral sinus or 



oral pit [stomodseum] and fore 



L'ut, and which has been de- 



Bcril eil >ince the time of REMAK 



Tig. 152. Human embryo t I 15 mm. 



long, neck measurement* Drawing from 

 a reconstruction, Menschliche 



Eiubr\'>i:i-:i ). Magnified 40 diameters. 



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



:.idtlle part <>f the \i-iitricle <>f : 

 Vc, vena cava superior or ductus Cu\ieri ; 

 Sr, si; unibilicalis ; 



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

 heart ; D, diaphragm ; T.om. vena otuphalo- 

 mesenteries ; Lb, solid fundament of the 

 liver ; L'-.i, lu-^itic duct. 



haut). By its rupture and the 

 degeneration of the shreds of it 

 known as the primitive pufnfnl 

 re/ "in communication with the 

 outside is established (Plate I., 

 iiir-. 4 and 7 m). 



In the case of the Chick the oral 

 pit is observable on the second day 

 of incubation, the front end uf the 

 i inlirvonic fundament having a short 

 time previously elevated it.^elf as a 

 cephalic knob above the extra-em- 



1'iyonic part of the irerm-layers. The rupture of the pharyngeal membrane 

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

 the pharyngral membrane is not yet ruptured. His l,a> studied in de-tail this 

 early sta<;e in Man on his embryo " Lg t " the age of which he estimates at twelve 

 days. 



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

 M } > ) presents a very uniform condition and appears as a large 



* [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.] 



