780 DEVELOPMENT OF THE ALIMENTARY CANAL 



bladder and the umbilicus, is consolidated into a rounded cord, which 

 is termed the urachus. 



The urinary bladder is at first, accordingly, a pyriform ' sac (Fig. 

 287, e), communicating at its base with the lower portion of the intes- 

 tinal canal, and continuous by its superior pointed extremity with the 

 solid cord of the urachus, by means of which it is attached to the inter- 

 nal surface of the abdominal walls at the situation of the umbilicus. 

 Afterward, the bladder loses this conical form, and its superior fundus 

 becomes in the adult rounded and bulging. 



Development of the Mouth and Face The intestinal canal is at first 

 a cylindrical tube, closed at its anterior as well as at its posterior 

 extremity. In the region of the abdomen, which in the earliest periods 

 of development constitutes nearly the whole length of the body, the 

 blastoderm separates, as previously described (p. 735), into two laminae, 

 an outer and an inner. The outer lamina, consisting of the external 

 integument and the subjacent voluntary muscles, forms the parietes of 

 the abdomen. The inner lamina forms the mucous membrane of the 

 alimentary canal, with its covering of involuntary muscular fibres. 

 Owing to the separation of these two laminae, there is formed the peri- 

 toneal cavity, between the intestine on the one side and the abdominal 

 walls on the other. 



But in the anterior part of the body of the embryo, this separation 

 between the two laminae of the blastoderm does not take place. Con- 

 sequently, the corresponding portion of the alimentary canal, namely, 

 the oesophagus, remains in contact with the surrounding parts ; and its 

 anterior rounded extremity, the pharynx (Fig. 287, d), lies immediately 

 underneath the head, covered in front only by the tissues of the external 

 blastodermie layer. 



At this time there are formed, on the sides and front of the neck, four 

 nearly transverse fissures, the cervical fissures, leading from the exte- 

 rior into the cavity of the pharynx. These fissures, or clefts, are analo- 

 gous to those which exist permanently at the same situation in fishes, 

 where the gills are located, and by which the water, taken in at the 

 mouth, is expelled through the sides of the neck. But in the mamma- 

 lian embryo they have only a temporary existence as continuous open- 

 ings. The three lower fissures disappear entirely by the subsequent 

 adhesion of their adjacent edges ; and in the chick, according to Foster 

 and Balfour, are completely closed by the seventh day of incubation. 

 The upper fissure is converted into a narrow canal, leading from the 

 exterior into the pharynx, but closed about its middle by a transverse 

 partition. The outer portion of this canal becomes the external audi- 

 tory meatus ; the inner portion, the Eustachian tube. The transverse 

 partition is the membrana tympani. 



The cervical fissures in man are especially connected with the forma- 

 tion of the mouth and face. Between the fissures there are, of course, 

 bands or ridges of solid tissue, belonging to the external lamina of the 

 blastoderm; and these bands, especially the upper, increase in growth to 



