SYSTEM. 



Ill 



head and those myotomes which form the muscles of the limb. 

 The process for the formation of the posterior fin is essentially 

 the same. ^ 



In the^ead region, although no little study has been de- 

 voted to the subject of the mesoderm segments, naturalists are 

 not in unison as to their results. Not 

 only is there a difference of opinion as to 

 the number of myotomes developed in 

 this region, from the nine recognized by 

 van Wijhe to the seventeen or more 

 claimed by Dohrn and Killian, but it is 

 even disputed whether these be true 

 somites. The questions involved will be 

 taken up in another place. Setting these 

 points aside, it may briefly be said that 

 in those forms which have been most 

 carefully studied (the elasmobranchs) 

 there are ten 1 myotomes developed in the 

 head region. Each of these which occur 

 in front of the ear is completely separated 

 from its fellows, a fact which leads some 

 to believe that we have to do here, not 

 with the whole mesothelial structures as 

 in the trunk region, but with merely the 

 myotome zone. This matter, however, 

 would seem to have less importance than 

 is sometimes given to it ; for we must re- 

 member that the vertebrate head is an 

 extremely complicated structure, all the 

 parts of which have been greatly modi- 

 fied, while the appearance of the gill slits 

 is of itself sufficient to explain the ab- 

 sence of a continuous metacoele. 



To the muscle fibres, the development of which was out- 

 lined above, other parts of mesenchymatous origin are added 

 in the development of the definitive muscle. This connective 



1 There is clearly one pair of coelomic cavities in front of the first recognized by van 

 Wijhe (Fig. 121, ac]. 



FIG. 121. Section- 

 through the head of em- 

 bryo Acanthias at about the 

 stage of Fig. 122. a, aoita; 

 ac, anterior (premandibu- 

 lar) head cavity ; e, pig- 

 mented epithelial layer of 

 eye; /, fore brain; gg 9 

 Gasserian ganglion ; h, 

 hind brain ; /, lens of eye ; 

 , notochord ; />, pharynx ; 

 r, retinal layer; I, 2, 3, 

 head cavities. 



