112 A. M. PATERSON. 



Several of these cells occur in one somite in a line from front 

 to back. In other words, the fibres are shorter than the 

 thickness of the somite. In transverse sections the fibres 

 appear rounded with large nuclei, and are more separated from 

 one another. The cells at the upper and lower ends of the 

 muscle-plate stain most deeply. 



The bud which gives rise to the fore limb has at this 

 date attained considerable size. It projects almost directly 

 outwards from the side of the trunk (fig. 2), growing partly 

 from the mesoblast above the angle between somatopleure and 

 splanchnopleure, and partly from the somatopleure itself. It 

 consists of a mass of mesoblastic cells, densely packed together, 

 especially at the surface and distal end. Towards the origin 

 of the limb the cells become more scattered. This mass of 

 mesoblast is covered by a layer of epiblast, which presents two 

 thickenings where the cells are in most active growth, one 

 forming a cap, covering the pointed outer end, the other forming 

 a ring round the root of the limb, and appearing in transverse 

 sections as thickenings above and below the root of the limb. 

 The mesoblastic cells are entirely undifferentiated as yet; they 

 are rounded, and stain deeply. Embryonic blood spaces are 

 found here and there. The position of the muscle-plate is 

 different from what has been described in the region between 

 the limbs. By the growth of the limb bud it has become 

 separated from the somato-splanchnopleuric angle (a.). Its 

 lower end reaches to about the centre of the upper half of the 

 base of the limb, that part which is continuous with the inter- 

 mediate cell mass. Moreover, the plate does not lie external 

 to the angle of the body cavity : otherwise it has the same 

 relative position as in the dorsal region to the parts which 

 constitute the trunk at this period. Histologically, also, it 

 has very similar characters. The differences are seen in the 

 centre of the plate. The inner stratum of longitudinally 

 arranged spindle-cells is thicker. It stains badly, and the 

 nuclei of the fibres are large and well defined. The outer 

 layer is thinner in the centre, becoming thicker when traced 

 towards the end of the plate. The cells of this layer stain 



