XIV ELEMENTS OF VERTEBRATE EMBRYOLOGY 453 



Before proceeding further with the mesoderm we may note that the 

 notochord originates from a longitudinal ridge of endoderm which 

 separates off in the mid-dorsal line (Fig. 189, C)j and that the central 

 nervous system originates as usual from a thickening of the ectoderm 

 (Fig. 189, A, m.p) which becomes converted into a neural tube by a 

 process differing slightly in detail from that characteristic of other 

 vertebrates (Fig. 189, B, C, D). 



Each mesoderm segment becomes subdivided (Fig. 189, D, right side 

 of fig.) into a ventral portion (lateral mesoderm) and a dorsal portion 

 (myotome). Of these the former gives rise to the lining of 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 muscle 

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

 coelomic cavity of the myotome becomes completely obliterated. Unlike 

 what happens in the case of the lateral mesoderm the myotomes retain 

 their individuality so that even in the adult the longitudinal muscles 

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

 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 rriore 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 of 

 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 



