ON THE FATE OF THE MUSCLE-PLATE. 113 



deeply. They are scattered in the centre, becoming more 

 closely packed at each end. 



In Chick embryos 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 are seen to be more pronounced. In the dorsal 

 region (fig. 5), owing partly to the increase in vertical extent 

 of the embryo, the muscle-plate {m.p.) has become elongated, 

 and at the same time thinned out. There is no distinct trace 

 of an outer layer to be found, except at the ends of the plate. 

 Here the cells retain their primitive character, and stain 

 deeply. The rest of the plate consists entirely of longitudinally 

 arranged fusiform cells. The muscle-plate has a peculiar 

 bend, passing almost vertically downwards towards the body 

 cavity, and then suddenly sweeping outwards to enter the body 

 wall. Its lower end has passed still farther down the somato- 

 pleure, lying close to the inner side. In the fore-limb at this 

 date (fig. 6) the muscle-plate retains its original position. It 

 does not extend outwards farther than the somato-splanchno- 

 pleuric angle. Its histological characters are the same as in 

 the dorsal region. The limb bud itself has increased in size, 

 and is now directed downwards. The cells which compose it 

 are still undifferentiated. 



In embryos at four days, in the regions of the trunk between 

 the limbs (fig. 7), the muscle-plate {m.p.) has extended down 



VOL. XXVIII, PART 1. NEW SER. H 



