210 EMBKYOLOGY OF THE LOWER VERTEBRATES CH. 



interest is that the anterior limit of this series of recognizable 

 segments agrees approximately ' with the anterior end of the 

 definitive notochord. 



In front of the " fourth " myotome of van Wijhe we find what 

 appear to be fairly typical third and second myotomes, each con- 

 tinuous ventrally with the wall of the pericardiac portion of the 

 splanchnocoele. Of these myotome III gives rise to the External 

 Rectus muscle and II to the Superior Oblique. At the front end of 

 the series we have the first or premandibular or oculomotor myotome, 

 peculiar in that it is fused with its fellow across the mesial plane 

 and that it no longer shows any connexion with the splanchnocoelic 

 mesoderm. It gives rise to the four eye-muscles supplied by the 

 Third cranial nerve the Superior, Internal, and Inferior Rectus, 

 and the Inferior Oblique. 



We have so far dealt only with the myotomes but the lateral 

 or splanchnocoelic mesoderm is also continued well forwards into the 

 head-region. Its more ventral portion forms the lining of the 

 pericardiac cavity, while its more dorsal portion becomes traversed 

 by . the visceral pouches or clefts. The splanchnocoelic mesoderm 

 ventral to myotomes II and III comes to form a stalk -like 

 connexion between the myotome and the pericardiac wall (Fig. 115). 

 This stalk is hollow in the case of myotome II and lies in the 

 mandibular arch: in the case of myotome III it is solid and lies in 

 the hyoid arch. In both cases the wall of the stalk gives rise to the 

 muscular apparatus of the particular arch in the one case the 

 masticatory muscles and in the other the hyoidean musculature 

 which is destined to attain to such a development in the mammals 

 as the musculature of the face. 



The splanchnocoelic mesoderm corresponding to the myotomic 

 mass behind myotome III (* in Fig. 115) is said to give rise to the 

 musculature of the branchial arches. As the myotomic mass in 

 question shrivels up during development, and the occipital myotomes 

 move forwards to take its place, these myotomes come to overlie 

 the splanchnocoelic mesoderm which gives rise to the branchial 

 muscles. Consequently as will be realized the position of myotomes 

 t, u, and v in relation to clefts III, IV, and V as shown in Fig. 115 

 is secondary, the myotomes having moved forwards before the 

 formation of these clefts. 



The above sketch has dealt with the cephalic mesoderm of 

 Miobranchs but a similar scheme of development with minor 

 variations in detail holds for other Vertebrates. Upon tin- wlmlr it 

 niav be said that with upward progress in the evolution of the 

 V. -rtcl.rjLta the segmentation of the mesoderm in the hinder part of 

 tin- head becomes more and more obscured. Ri^ht up to the highest 

 forms liou.-ver traces of it persist. In Fowl embryos of aUmi the 

 third day of incubation tin; series of nhvimis mynlnim's may often 

 9611 tO !'" prolnn-e.l forward (866 Fig, 236) I'.v faintly visible 

 agreeing in size and exactly in series \\ith ihe m\ot<>mes. 



