xiv ELEMENTS OF VERTI hi ! I ; K YOLOGY 453 



Before proceeding further with tl.. ,1 t|, r 



notochord originates from u longitudinal nd-c of endoderm \vhi h 

 separates off in the mid-dorsal line (l-'i-. i.S,. ( j. and that the central 

 nervous system originates as usual from a thickcmnj o! the ed 

 (Fig. iSo, A, ;;/./)) which becomes converted into a neural tul" 

 process differing slightly in detail from that .1 other 



vertebrates (Fig. ,s,,. I',. (', D). 



Each mesoderm segment becomes subdivided (! ; side 



of fig.) into a ventral portion (lateral nu-x.derm) and a dursil p 

 (myotome). Of these the former gives rise to the lining ol the splanchno 

 coele, the cavities of successive segments opening into one another by 

 the disappearance of the intervening walls. By extension downwards to 

 the mid-ventral line the splanchnocoele becomes eventually continuous 

 across the mesial plane ventral to the alimentary canal. 



The inner wall of the myotome becomes much thickened and is 

 eventually converted into a thick mass of longitudinally-running inux-le 

 fibres which retains the name myotome in the adult. In this process tin 

 coelomic cavity of the myotome becomes completely obliterated. Unlike 

 what happens in the case of the lateral mesoderm the myo tomes retain 

 their individuality so that even in the adult the longitudinal muscles 

 consist of distinct blocks one behind the other. Originally the mvotomcs 

 lie completely dorsal to the lateral mesoderm but as development pro- 

 ceeds they extend down towards the mid -ventral line, insinuating 

 themselves between the lateral mesoderm and the outer skin and so 

 muscularizing the more ventral parts of the body-wall. 



The more typical vertebrates are distinguished from Amphioxus by 

 the fact that the egg contains a much greater amount of yolk and is 

 consequently of much greater size. This yolk is so much dead inert 

 material which tends to clog and obstruct the living activities of the 

 cytoplasm. Consisting of greatly condensed food-material it is ot 

 relatively high specific gravity and tends to gravitate towards the lower 

 or abapical pole of the egg : consequently we find that its interference 

 with the normal processes of development is least in the neighbourhood 

 of the apical pole and greatest towards the opposite pole, where its 

 interference is more marked in accordance with the greater proportion 

 it bears to the living cytoplasm in which it is embedded. 



The effects produced upon the early stages of development by an 

 increased proportion of yolk in the abapical portion of the egg are well 

 illustrated by the three ganoid eggs shown in Fig. 190. In the Sturgeons 

 (Fig. 190, A) the egg measures from 2 mm. to 2-8 mm. in diameter. The 

 cytoplasm in the neighbourhood of the apical pole has disseminated 



