210 EMBEYOLOCIY OF THE LOWEK VEKTEBEATES 'oh. 



'interest is that the anterior limit of this series of recognizable 

 jsegments 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 

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

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

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

 I 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 Eectus, 

 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 



1 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 

 i 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 



Elasmobranchs but a similar scheme of development with minor 



variations in detail holds for other Vertebrates. Upon the whole it 



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



Vertebrata the segmentation of the mesoderm in the hinder part of 



the head becomes more and more obscured. Eight up to the highest 



forms however traces of it persist. In Fowl embryos of about the 



i third day of incubation the series of obvious myotomes may often 



t be seen to be prolonged forward (see Fig. 236) by faintly visible 



i blocks agreeing in size and exactly in series with the myotomes. 



