iv MESODEEM OF HEAD 209 



region and (2) in the retention, to a greater extent than in the 

 trunk, of the muscle-forming capacity of that part of the mesoderm 

 which lies ventral to the myotomes. 



The mesoderm of the head-region shows the least amount of 

 modification posteriorly where its relation to the mesoderm of the 

 trunk is still clear. In the occipital region — the region between 

 the otoeyst and the occipital arch, which may be taken as the 

 hinder limit of the skull — we find a series of typical (" occipital " — 

 Fiirbringer) myotomes, the mesoderm ventral to which takes part in 

 the lining of the splanchnocoele just as in the trunk-region. This 

 series of occipital myotomes seems clearly to be undergoing a 

 process of reduction. It is largest in such, comparatively primitive 

 forms as Elasmobranchs.. Again during ontogenetic development 

 the series commonly shows progressive reduction. In Spinax for 

 example seven occipital myotomes make their appearance7~blit as 

 development goes on the anterior three (t, u, v) i break up and dis- 

 appear, the fourth (w) does so incompletely, while the last three 

 (x, y, z) develop into definite muscle-segments though of small size. 

 As each anterior myotome disappears those behind it become shifted 

 forwards so that its place becomes occupied by the myotome 

 originally behind it in the series. It will be realized that there is 

 thus introduced a serious source of possible error which has to be 

 carefully borne in mind in observations on the development of the 

 occipital region where the identification and correct reference of 

 individual myotomes to their place in the series is of importance. 



Anteriorly the series of occipital myotomes is prolonged forwards 

 past the otoeyst by a mass of mesoderm (* in Fig. 115) which was 

 regarded by van Wijhe (1883) — who may be said to have laid the 

 foundations of modern work upon the segmentation of the mesoderm 

 of the head — as the equivalent of a single (" fourth ") myotome. It 

 has already been indicated that the series of occipital myotomes is 

 undergoing reduction from its front end backwards and it seems on 

 the whole more probable that van Wijhe's " fourth " myotome in the t 

 Gnathostomata is to be interpreted not as a single myotome but 1 

 rather as the degenerate remnant of a series of myotomes: The 

 number of myotomes Originally present in this series does not 

 appear to be capable of decision with any degree of certainty. 

 Possibly it was very considerable and Froriep finds even in 

 ontogeny {Torpedo) that during early stages (Stage " D " of 

 Balfour) as many as six distinct segments are recognizable in the 

 region in question — in other words that the series of myotomes 

 commences not with t but^with^w^ the anterior members of the 

 series disappearing in turn as development proceeds. A point of 



1 The hind end of the series — the occipital arch — being taken as a fixed point 

 while the front end varies, Fiirbringer has introduced the convenient method of 

 designating the individual occipital myotomes (or their nerves) by the terminal 

 letters of the alphabet — the last being z, the one next in front y, and so on. The 

 myotomes behind the occipital arch are counted as belonging to the trunk and are 

 designated by numerals 1, 2 and so on (cf. Fig. 220). 



VOL. IT P 



