MYOTOMES OF THE HEAD. 429 



Mesoblast of the Head. 



Body Cavity and Myotomes of the Head. During stage F the 

 appearance of a cavity on each side in the mesoblast of the head 

 was described. (Vide PI. 10, figs. 3 b and 6//.) These cavities 

 end in front opposite the blind anterior extremity of the alimen- 

 tary canal ; behind they are continuous with the general body- 

 cavity. I propose calling them the head-cavities. The cavities 

 of the two sides have no communication with each other. 



Coincidently with the formation of an outgrowth from the 

 throat to form the first visceral cleft, the head-cavity on each 

 side becomes divided into a section in front of the cleft and a 

 section behind the cleft (vide PI. 15, figs. 4 a and ^b pp.); and 

 during stage H it becomes, owing to the formation of a second 

 cleft, divided into three sections: (i) a section in front of the 

 first or hyomandibular cleft ; (2) a section in the hyoid arch 

 between the hyomandibular cleft and the hyobranchial or first 

 branchial cleft ; (3) a section behind the first branchial cleft. 



The section in front of the hyomandibular cleft stands in a 

 peculiar relation to the two branches of the fifth nerve. The 

 ophthalmic branch of the fifth lies close to the outer side of its 

 anterior part, the mandibular branch close to the outer side of its 

 posterior part. During stage I this front section of the head- 

 cavity grows forward, and becomes divided, without the inter- 

 vention of a visceral cleft, into an anterior and posterior division. 

 The anterior lies close to the eye, and in front of the commencing 

 mouth involution, and is connected with the ophthalmic branch 

 of the fifth nerve. The posterior part lies completely within the 

 mandibular arch, and is closely connected with the mandibular 

 division of the fifth nerve. 



As the rudiments of the successive visceral clefts are formed, 

 the posterior part of the head-cavity becomes divided into suc- 

 cessive sections, there being one section for each arch. Thus 

 the whole head-cavity becomes on each side divided into (i) a 

 premandibular section ; (2) a mandibular section ; (3) a hyoid 

 section ; (4) sections in the branchial arches. 



The first of these divisions forms a space of a considerable 

 size, with epithelial walls of somewhat short columnar cells. It 



