316 EMBRYOLOGY. 



the germinal membrane by a considerable space. The inferior 

 layer of the serous ventral lamina forms the ventral paries, and 

 gives origin to the bones and muscles which compose the neck 

 and trunk. Inferiorly, the vascular lamina lies upon it, and 

 this, with the serous lamina, evolves the formations which are 

 now to be described. On either side, under the vertebral 

 column, there is a lamina detached, which grows thicker, and 

 increases in a direction perpendicularly downwards ; these are 

 the lamina mesenteries, between which there is, at first, an 

 open triangular-shaped channel or cleft, the foramen mesen- 

 terii ; both the mesenteric laminae push the mucous layer be- 

 fore them, and speedily unite, at an acute angle, in the suture 

 (fig. 343, A, h, B, m). The furrow, or foramen of the mesen- 

 tery, resembles an equilateral triangle, with one of its angles 

 pointing directly downwards. After the union of the two 

 mesenteric laminae, the resulting structure grows most rapidly 

 posteriorly, opposite the middle of the body, and here forms a 

 septum, dividing the abdominal cavity into two halves. 



It is at the beginning of the intestinal canal, where the ven- 

 tral laminae are converging, that the branchial arches are deve- 

 loped ; the parietes of the body here become thinner ; and in 

 this, the cervical region, several clefts or fissures make their 

 appearance, which sink downwards, and penetrate through 

 the mucous layer ; there are three pairs, or, with the oral 

 aperture, four pairs of such fissures, but the posterior pair are 

 extremely small ; they are called the branchial fissures fissurae 

 branchiales ; between them lie three segments, or divisions of 

 the ventral laminae, which are blunt and rounded anteriorly, 

 bevelled off towards the digestive cavity, and therefore sickle- 

 shaped ; these are named the branchial arches arcus bran- 

 chiales (figs. 339, 340, 341, 343, &c.) ; the fourth branchial 

 arch is placed hindmost, and is not yet distinct from the ven- 

 tral lamina. On the fourth day, the two most anterior bran- 

 chial arches increase in thickness (fig. 341, between # 4 and 

 ff 2 ) ; a new fissure is formed posteriorly (fig. 347, ff } ) ; on the 

 fifth day, the foremost fissure closes (fig. 342, A, between d 

 and e),'and the foremost branchial arch unites with its fellow 

 of the opposite side, and forms the lower jaw (fig. 342, A, d, 

 B, e) ; the next in succession is transformed into the os hyoides 

 (fig. 342, A, e, B, /). The two last branchial fissures close 



