156 ALIMENTAKY CANAL 



development of the olfactory nerves is in much closer accordance with what is 

 known of the structure of the olfactory epithelium and of the olfactory bulb in the 

 adult animal. 



The olfactory nerve-fibres are seen at a very early stage running between the 

 brain and the olfactory pit. They are at first connected (Mihalkovics) with 

 every part of the epithelium derived from the ectoderm of the pit ; but it is only in 

 the upper part of the fossae that the permanent connexion by nerve-fibres with 

 the olfactory lobe persists. In the lower parts of the fossae the epithelium remains 

 thinner, loses its nerve- connexions, and becomes ciliated, the fossae assuming the 

 role merely of respiratory passages. 



DEVELOPMENT OF THE ALIMENTARY CANAL. 1 



The early stages in the development of the alimentary canal have already been 

 described in treating of the formation of the embryo (p. 52 seg.). We resume here 

 at a phase reached during the third week, in which the primitive tract has assumed 

 the condition of a tube, formed by the folding-in of the splanchnopleure, and 

 consisting of an anterior section (fore-gut), a posterior section (hind-gut), and a middle 

 section (mid-gut), continuous with the cavity of the umbilical vesicle (yolk-sac). 

 The fore-gut is still closed in front by the buccopharyngeal membrane ; while the 

 hind-gut is separated from the surface by the primitive cloacal membrane, formed 

 from the persistent part of the primitive streak. From the hind-gut, further, the 

 allantoic diverticulum extends as a narrow tube into the body-stalk. We must 

 now consider the development of the organs derived from the several sections of 

 the primitive alimentary canal ; but it will be convenient to consider in the first 

 instance the formation of the buccal cavity. 



DEVELOPMENT OF THE MOUTH. 



We have already seen (p. 52) that at a very early stage, while the blastoderm 

 is still a flattened plate, there is an area between the head end of the axis and the 

 cross portion of the ccelom, in which the ectoderm and entoderm are applied to one 

 another without any intervention of mesoderm. This has been named the bucco- 

 pharyngeal membrane. When the head-fold is developed and the fore-gut formed, 

 the membrane is necessarily bent in below the head end of the embryo. It forms 

 the floor of a wide and shallow fossa, bounded in front by the down-bent fore-brain, 

 and behind by the pericardium, and constitutes a septum between this fossa, which 

 is the stomodoeum, and the fore-gut (fig. 198). This stage is reached by the twelfth 

 day, but during the third week the depression is converted into an actual chamber 

 by the forward growth of the fore-brain and by the development of the prominences 

 which ultimately form the face. These prominences are the fronto-nasal which 

 overlaps the depression above and in front, the mandibular arches which bound it 

 behind, and the maxillary processes which close it in on each side. By the end of 

 the third week (in embryos of 3*2 mm., His), the buccopharyngeal membrane becomes 

 broken through so as to establish a communication between the stomodoeum and 

 fore-gut (fig. 199) ; but before this opening is effected, a pocket is developed 

 immediately in front of the septum, which extends upwards in the angle between 

 the fore-brain and hind-brain formed by the cephalic flexure. This recess (Kathke's 

 pocket) comes presently into connexion with a projection from the floor of the 

 fore-brain, and forms with it the pituitary body (see p. 115). The remains of the 



1 For literature of the alimentary tract and its gland, see to date of its publication, Maurer, Hertwig, 

 Handbuch der Entwickelungslehre II. Th. pp. 241, seq. ; of the respiratory tract ib. p. 105 seq. ; of the 

 mouth ib. p. 35 ; of the tongue, ib. p. 53. 



