106 A. M. PATERSON. 



Towards the origin of the limb the cells become more scattered. 

 This mass of mesoblast is covered by a layer of epiblast, which pre- 

 sents 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 meso- 

 blastic 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 

 intermediate 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 

 deeply. They are scattered in the centre, becoming more closely 

 packed at each end. 



In Chick embrj^os at three days six hours (Fig. 3), in the dorsal 

 region the muscle-plate (m. p.) has extended a short distance down 

 the body wall in the somatopleure. The central portion of the 

 plate is thicker, owing to an accumulation of the longitudinal fibres 

 of the inner layer. The cells of the outer stratum are still more 

 separated in the centre, and do not stain so deeply. The layer as a 

 whole is thinner. The ends of the plate still present the primitive 

 condition of the cells, which are angular, packed close together, and 

 stained deeply. In the region of the fore limb at this date (Fig. 4) 

 the muscle-plate (m. p.) has the same characters ; but its position is 

 different. Its lower end reaches no farther than midway between 

 the dorsal attachment of the limb to the trunk, and the somato- 

 splanchnopleuric angle. 



Twelve hours later (at three days eighteen hours) these changes 



