I08 MORPHOLOGY OF THE ORGAXS OF VERTEBRATES. 



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 vertebras, they were formerly called proto- 

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

 ing to the skeletal structures, but gi\e 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 raesal 

 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 ma}' 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 fibrillas, 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. ii6. Myo- 

 tomes of Amblystoma 

 in process of conver- 

 sion intomuscle-plates. 

 c, remains of myo- 

 coele ; ck, chorda ; c, 

 epidermis ; ot, muscle 

 developing from inner 

 plate of myotome; c, 

 outer plate of myo- 

 tome ; s, skelelogenous 

 tissue. 



