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MORPHOLOGY OF THE ORGANS OF VERTEBRATES. 



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cubical hollow bodies on either side of the notochord and cen- 

 tral nervous system. From the early idea that these bodies 

 gave rise to the vertebrae, they were formerly called proto- 

 vertebrae. We now know that they contribute little or noth- 

 ing to the skeletal structures, but give rise to the voluntary 

 musculature of the body. The processes involved in the con- 

 version of these epithelial walls into muscle the histogenesis 

 of muscle must be traced first. 



In the majority of the vertebrates the cells of the mesal 

 wall (i.e., that towards the notochord and nervous system) rap- 

 idly increase in number, thus obliterating 

 the myoccele. In this process the cells lose 

 their original shape and arrangement as a 

 cylindrical epithelium, and form elongated 

 cylinders, the axes of which are parallel to 

 the longitudinal axis of the body. Each of 

 these primitive muscle cells at first contains 

 but a single nucleus ; but by division several 

 arise, which may either eventually lie in 

 the centre (amphibia) or on the periphery 

 (mammals) of the cell. At the same time 

 the peripheral protoplasm of the cell be- 

 comes differentiated into numbers of fine 

 longitudinal fibrillae, which increase in num- 

 ber so that at last all except a small amount 

 of protoplasm in the immediate vicinity of 

 the nucleus has been converted into these 

 contractile structures, the epithelial cell 

 becomes a muscle fibre. The lateral or 

 outer wall of the myotome does not par- 

 ticipate in this muscle formation, but is said 

 to give rise to the deeper layer (corium or 

 derma) of the skin. The process of the histo- 

 genesis of muscle in the cyclostomes differs 

 in some particulars from that given above. 

 The myotomes, after their separation from the mesothelial 

 tissues, increase rapidly in their dorso-ventral dimensions, and 

 gradually push in between the lateral plate and the ectoderm in 



FIG. 116. Myo- 

 tomes of Amblystoma 

 in process of conver- 

 sion intomuscle-plates. 

 f, remains of myo- 

 coele ; ck, chorda ; <?, 

 epidermis ; /, muscle 

 developing from inner 

 plate of myotome ; 0, 

 outer plate of myo- 

 tome ; s, skeletogenous 

 tissue. 



