VII.] THE MUSCLE-PLATES. 211 



Shortly after the formation of the ligamentum suspensorium 

 the remaining cartilage of the intervertebral segments is con- 

 verted into the meniscus between each two vertebrae, and in the 

 tail into the annulus fibrosus. Both are absent in the sacrum. 



Muscle-plates. We shall conclude our account of 

 the mesoblastic somites 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 somato- 

 pleure and splanchnopleure. On the fourth day how- 

 ever (Fig. 68 mp.) they extend 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 con- 

 verted has been somewhat disputed. Some embryolo- 

 gists have stated that they only form the muscles of 

 the back. We have, however, little doubt that all the 

 episkeletal muscles, to use Professor Huxley's term 

 (Vertebrates, p. 46), are their products; a view also 

 adopted by Professors Huxley and Kolliker. 



The development of the subvertebral system of muscles 

 (hyposkeletal of Huxley) has not been worked out, but on the 

 whole there is reason to believe that it is derived from the 

 muscle-plates. Kolliker, Huxley and other embryologists believe 

 however that these muscles are independent of the muscle-plates 

 in their origin. 



Whether the muscle of the diaphragm is to be placed in the 

 same category as the hyposkeletal muscles has not been made out. 



It is probable that the cutaneous muscles of the trunk are 

 derived from the cells given off from the muscle-plates. Kolliker 

 however believes that they have an independent origin. 



142 



