DEVELOPMENT OF THE ALIMENTARY TRACT. 



1695 



The entrance into the primary oral cavity is a pentagonal opening bounded by 

 five projections, superiorly by the unpaired frontal process, extending downward 

 from the region of the anterior cerebral vesicle, laterally by the maxillary processes, 

 and inferiorly by the fused mandibular processes of the first visceral arches (Fig. 74). 

 The further changes leading to the formation of the definitive mouth and the sepa- 

 ration of the oral and nasal cavities are described in connection with the development 

 of body-form (page 59). 



The primitive pharynx bears on each side a series of four lateral dilatations, the 

 pharyngeal pouches (Fig. 73), corresponding to the inner half of the visceral clefts 

 seen in water-breathing animals. In the mammals true fissures are not formed, the 

 visceral clefts being represented by the external and internal furrows lying between 

 the visceral arches and separated by a delicate ecto-entoblastic partition. The details 

 of the development and metamorphosis of the visceral arches and furrows have been 

 considered (page 60). 



FIG. 1429. 



Posterior cerebral vesicle 

 Primitive pharynx 



Ventral aorta 



Gut-tube 



Neural tube 



Middle cerebral vesicle 



Anterior cerebral vesicle 



Pharyngeal membrane 



Oral recess (stomodaeum) 

 Truncus arteriosus 

 Pericardia! sac 

 Primitive auricle 



Primitive ventricle 



Vitelline duct 



Caudal pole (obliquely cut) 



Sagittal section of early rabbit embryo, showing oral recess and primitive pharynx still separated. X 12. 



Formation of the Anus. The posterior or caudal segment of the primitive 

 gut-tube is the seat of the changes leading to the formation of the excretory orifice. 

 Formerly the development of the anus was regarded largely as the repetition of a 

 process similar to that leading to the communication between the oral recess and the 

 fore-gut, an external depression (proctodatum} being separated from the hind-gut by 

 an ecto-entoblastic partition which later was broken down to form the anus, which was 

 considered a new structure. 



The studies of Gasser, Kupffer, Bonnet, Hertwig, and others have emphasized the 

 close relations between the anus and the blastopore. According to these investigations, 

 the blastopore probably gives rise to the transient neurenteric canal, while behind the 

 latter lies the rapidly proliferating tissue of the primitive streak. When the primitive 

 streak is regarded as the fused and elongated blastopore (page 25), it follows that 

 the anlage for the anus is located in the posterior part of that structure, and, further, 

 that the primary position of the anal anlage is on the dorsal surface of the embryo. 



