VI.] THE MUSCLE-PLATES. 159 



16. We shall conclude our account of the proto vertebrae 

 by describing the changes which take place in the muscle- 

 plates. 



In the chick these are somewhat complicated, and have 

 not been fully worked out. 



On the third day the muscle-plates end opposite the 

 point where the mesoblast becomes split into somatopleure 

 and splanchnopleure. On the fourth day however (Fig. 

 47 mp.) they extend to a certain distance into the side walls 

 of the body beyond the point of the division into somatopleure 

 and splanchnopleure. 



Into what muscles of the trunk they become converted has 

 been somewhat disputed. There is no doubt that it is only epi- 

 skeletal muscles, to use Professor Huxley's term {Vertebrates, 

 p. 46), that are derived from them, but some embryologists 

 have stated that they only form the muscles of the back. 

 We have, however, little doubt that all the episkeletal 

 muscles are their products ; a view also adopted by Professors 

 Huxley and KoUiker. 



According to KoUiker the muscle-plates give rise to (i) the deep dorsal 

 muscles, such as the semispinalis multifidus &c., and (2) the visct-ral muscles 

 as represented by abdominal muscles, the muscles of tbe breast, the superficial 

 muscles of the neclc, and the muscles of the jaws and face. 



Tbe front dorso-lateral (hyposkeletal) muscles, according to KoUiker, are 

 derived from a front (ventral) muscle-plate, which is formed from the most 

 ventral portion of the protovertebrte, but is very limited in extent in the 

 fowl. These muscles include the longus colli, the recti antici, and quadratus. 



This view differs from that of Huxley, chiefly in considering only the 

 ventral dorsal muscles as hyposkeletal, and not also the inner visceral muscles. 

 Huxley believes that all the episkeletal muscles are derived from the muscle- 

 plates, but does not give an opinion as to the cells of the embryo from 

 which the hyposkeletal muscles take their origin. 



His takes an entirely different view; he believes that the muscles of the back 

 only are derived from the muscle-plates, but that the muscles of the sides and 

 ventral walls of the body are formed from the mesoblast of the somatopleure. 



There can be little doubt that the intrinsic muscles of the limbs are not out- 

 growths from the muscle-plates, but are formed independently in the meso- 

 blaslic tissues of which the limbs are composed. 



The origin of the extrinsic limb-muscles is not so certainly known. 



The cutaneous muscles are obviously derived from the original mesoblast of 

 the somatopleure. 



It seems very probable (though the subject has not yet been worked out) 

 that the hyposkeletal voluntary muscles underlying the vertebral column aie 

 derived from the intermediate cell-mass, which originally lies externally to the 

 protovertebrEe, but into which, as we have before said, the cleavage of the 

 mesoblast does not extend. 



In the first instance, as is clear from their mode of origin, the muscle-plates 

 correspond in number with the protovertebree, and this condition is permanent 



