THE MUSCLE-PLATES. 369 



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the ends of the muscle-plates retain unaltered their primitive 

 undifferentiated character, and the separation between them and 

 the surrounding connective-tissue cells is very marked. This 

 however ceases to be the case in the parts of the muscle-plates 

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

 canal; the thinnest sections and most careful examination 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 between 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 PI. 13, figs, i 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 (shewn in PI. 13, 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 protovertebrae, 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 

 PI. 13, 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 observations 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-plate arc concerned in 

 forming the great lateral muscle, though the splanchnic layer is 

 converted into muscles very much sooner than tJie somatic^. 



1 The difference between Dr Gb'tte's account of the development of the muscles 

 and my own consists mainly in my attributing to the somatic layer of the muscle-plates 



