256 Dr Allen Thomson m the Vascular System 



which a series of analoojies of the most interesting nature has been 

 established between air-breathing animals, and those which are 

 aquatic during the whole or some part of their existence *. 



From the manner in which the rudimentary intestinal canal 

 is formed) it has been shewn that the anterior and posterior 

 parts of the intestine constitute at first two shut sacs, into which 

 the only entrance is by a large opening in the middle between 

 them, by which they communicate with the sac of the yolk. 

 Neither mouth nor anus, therefore, at first exist ; but both these 

 apertures iare afterwards formed by a wasting away or absorption 

 of the substance of the germinal membrane, at the two extremi- 

 \ ties of the rudimentary intestine. The opening of the mouth 

 (Fig. 21 , e) appears towards the end of the second day of in- 

 cubation, some days before that of the anus is perforated. 

 A The mouth, or anterior opening into the intestinal canal -|-, 

 ,, has at first the appearance of a transverse slit, or cleft, on the 

 r lower part of tlie head. On the third day, the part of the in- 

 testine into which this opening leads becomes wider anteriorly, 

 and assumes the form of a cone, the apex of which is directed 

 towards the tail of the embryo. The walls of this cavity, which 

 .corresponds in many respects with the branchial cavity of carti- 

 laginous fishes, and with part of what is afterwards converted into 

 the pharynx of the bird, become thicker and of a firmer consistence 

 at the same period. Towards the end of the third diay six clefts, 

 i or transverse slits, make their appearance behind the mouth, three 

 on each side of the intestine. The foremost pair of these clefts 

 appears first, and the second and third after it gradually. The 

 :. wall of the pharyngeal cavity projects slightly at the parts be- 

 ; , tween the clefts on each side ; it is here of a firmer consistence 

 than elsewhere, and has the appearance of being formed of trans- 

 ^ verse bands, united anteriorly on the mesial line, like the branchial 



'^ • See the Memoirs of Rathke in the Repert. Giner. d'Anat. et de Physiol., 

 . . torn, vii., in the Edinburgh Medical and Surgical Journal, 1830; and in the 

 ^, Isis, 1825, No. 6, and 1828, No. 1 ; those of Buschke in thelsis, 1827, No. I, 

 and 1828, No. 2 ; and those of Baer in Meckel's Archiv. vol. ii., No. 4 ; and 

 in Breschet's Repertoire, 1829 j also Baer, de ovi raammalium et hominis 

 ^- engesi Eplstola. 



o if TTie evening alluded to can scarcely with correctness be called the 

 mouth at this period, this cavity being afterwards formed before it, by the 

 growth of the superior and inferior maxillae. 



