148 THE MUSCLE-PLATES. 



plates on a level with the notochord and lower part of the 

 medullary canal; the thinnest sections and most careful exami- 

 nation are needed to elucidate the changes taking place in this 

 region. The cells which form the somatic layer of the muscle- 

 plates then begin to elongate and become converted into muscle- 

 cells, at the same time that they are increasing in number to 

 meet the rapid demands upon them. One result of these changes 

 is the loss of the original clearness in the external boundary be- 

 tween the muscle-plates and the adjoining connective-tissue 

 cells, which is only in exceptional cases to be seen so distinctly as 

 it may be in PL xii. fig. 1 and 8. Longitudinal horizontal sections 

 are the most instructive for studying the growth of the muscles, 

 but transverse sections are also needed. The interpretation 

 of the transverse ones is however rendered difficult, both by 

 rapid alterations in the thickness of the connective-tissue layer 

 between the skin and the muscle-plates (shown in PL xii. fig. 8), 

 and by the angular shape of the muscle-plates themselves. 



A careful study of both longitudinal and transverse sections 

 has enabled me to satisfy myself of the fact that the cells 

 of the somatic layer of the protovertebrse, equally with the 

 cells of the splanchnic layer, are converted into muscle-cells, and 

 some of these are represented in the act of undergoing this 

 conversion in PL XIL fig. 8 ; but the difficulty of distinguishing 

 the outline of the somatic layer of the muscle-plates, at the 

 time its cells become converted into muscle-cells, renders it 

 very difficult to determine whether any cells of this layer 

 join the surrounding connective tissue. General considerations 

 certainly lead me to think that they do not ; but my observa- 

 tions do not definitely settle the point. 



From these facts it is clear, as was briefly stated in the 

 last chapter, that both layers of the muscle-jdate are concerned 

 in forming the great lateral muscle, though the splanchnic layer 

 is converted into muscles very much sooner than the somatic^. 



^ The difference between Dr Gotte's account of the development of the 

 muscles and my own consists mainly in my attributing to the somatic layer of 

 the muscle-plates a share in the formation of the great lateral muscles, which he 

 denies to it. In an earlier section of this Monograph, pp. 115, 116, too much 

 stress was unintentionally laid on the divergence of our views; a divergence 

 which appears to have, in part at least, arisen, not from our observations 

 being opposed, but from Dr Gotte's having taken the highly differentiated 

 Eombinator as his type instead of the less differentiated Elasmobranch. 



