iv MYOTOMES OF HEAD 211 



These blocks m.iv be indistinguishable in ordinary thin sections but 

 quite distinct in stained preparations of the whole embryo. It will 

 require strong evidence to justify the refusal to ^ivo them the inter- 

 pretation that at once suggests itself that these slight condensations 

 of tin; mesenchynie are as it were the ghostly remnants of once 

 existing myotomes which in Birds have ceased to become functional. 



An important side issue of their presence to be borne in mind is 

 that the slightly greater resistance of the more condensed portions 

 of mesenchynie must necessarily exercise pressure upon the soft 

 surface of the rapidjy growing brain and produce a modelling of its 

 surface which may be adequate to explain at least some of these 

 appearances of segmentation of the brain-region which are included 

 under the term neuromery. 



The blocks in question extend well forwards in the specimen 

 figured (Fig. 236) there are four distinguishable anterior to the 

 middle of the otocyst and they may be taken as additional evidence 

 in favour of there being not one but a number of myotomes repre- 

 sented in the region of van Wijhe's " fourth " myotome. 



It is of interest to note that in the Lampreys the blurring of 

 the segments immediately posterior to the third of van Wijhe's series 

 seems not yet to have come about and there is an undoubted simple 

 " fourth " myotome (Koltzoff, 1901). We may justifiably associate 

 this with the low degree of cephalization in these creatures which 

 has involved a persistence of, or more probably a reversion to, an 

 apparently archaic condition of this myotome and its immediate 

 successors in the series. 



The relations of segments I, II and III to the eye-muscles have 

 been worked out in a number of Elasmobranchs and similar conditions 

 have been described in Eep tiles and Birds. Our knowledge of the 

 holoblastic Vertebrates in this respect is still fragmentary. In the 

 case of Lepidosiren and Protopterus the eye-muscles develop out of 

 compact masses of mesenchynie in which it is impossible to recognize 

 definite segments (Agar, 1907) while on the other hand in Ceratodus 

 (Gregory, 1905) these segments make their appearance much as in 

 Elasmobranchs. 



Before leaving this part of the subject it should be pointed out 

 that not all morphologists are convinced that segments I, II and 

 III are actually serially homologous with the undoubted mesoderm 

 segments or myotomes of the trunk-region : the blurring of the 

 mesoderm arrangements between them and the admitted myotomes, 

 and more especially their late appearance in ontogeny, at a time when 

 the anterior members of the occipital series have already degenerated, 

 are. brought as evidence against the more generally accepted view. 

 The present writer does not feel inclined to attach great weight to 

 these objections. (1) The break or blurring of the series immediately 

 behind III seems adequately explained by the disappearance of 

 functional muscles in this region and (2) the relatively late appear- 

 ance of myotomes I to III is explicable by the fact that the 



