ON THE JPATE OF THE MUSCLE-PLATE. 115 



separated from one another by a small amount of intercellular 

 substance. Towards the dorsal and ventral surfaces of the 

 limbs the cells are more scattered, so that the central portion 

 of the limb in transverse sections appears darker than the 

 superficial parts. At the free end of the limb (4) there is still 

 no differentiation of tissue elements. The cells here form a 

 simple mass, without any distinction into layers or groups. 



In a five days' embryo the condition of the muscle-plate in 

 the region between the limbs (fig. 13, m. p.) is much the same 

 as at the last-mentioned period. It shows still further exten- 

 sion in a ventral direction down the body wall. In the same 

 embryo, in the regions of the limbs the muscle-plate (fig. 14, 

 m.p.) has clearly no connection with the muscular system of 

 the limb itself. It consists now of elongated fibres, forming, 

 in transverse sections, a column lying just outside the spinal 

 cord and nerves, and separated from the surface of the body 

 and from the limbs by a considerable thickness of ordinary 

 blastema. The several tissues of the limb are formed from 

 mesoblastic elements, developed in situ. A central core of 

 cellular cartilage (3) is very evident at this date. The cells 

 are arranged regularly in a concentric manner in transverse 

 sections ; in transverse rows in longitudinal sections. This 

 cartilaginous cylinder is found in longitudinal sections to be 

 broken up into segments, corresponding to the skeletal ele- 

 ments of the limb. The intercellular substance between the 

 cells has largely increased in amount. At the periphery of 

 this central mass the cells gradually become changed in cha- 

 racter, being more deeply stained in mass, and individually 

 becoming oatshaped or fusiform. The long axes of the cells 

 are directed from within outwards. Two layers of cells thus 

 appear, one above, the other below the cartilaginous bar. The 

 cartilage, however, is not yet distinctly marked off, but is con- 

 nected to these groups of cells by a definite and still more 

 deeply-stained (perichondrial) layer. The nerves to the limbs 

 derived from the plexuses have a very definite relation to these 

 central groups of cells, which are enclosed within the nerve- 

 trunks. The dorsal nerves pass over, and the ventral trunks 



