ON THE FATE OF THE MUSCLE-PLATE. 107 



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-splanchnopleuric 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 through 

 a third of the length of the body wall, lying close to the outer side 

 of the body cavity. Its relations and structure are the same as 

 before. Each end is surmounted by a cap of cells, which retain their 

 primitive characters ; rounded, fusiform, or angular, they stain 

 deeply, and are plainly separated off from the main part of the 

 muscle-plate. These cells can be traced on to the outer surface of 

 the muscle-plate, where they gradually become lost. The main part 

 of the plate consists of elongated fusiform cells. In the region of 

 the fore limbs (Fig. 8) the mesoblastic tissue of the limb still pre- 

 sents the same characters. The cells stain deeply, are round, ovoid, 

 and often multinucleated; but still undifferentiated. The fcetal 

 vessels are better marked. The muscle-plate (m. p.) occupies its 

 primitive position, ending below at the root of the limb. It has the 

 same structural characters as in the dorsal region ; but the undiffer- 

 entiated cells, which stain deeply, are best marked at the upper end 

 of the plate. 



In embryos at four days twelve hours, in the trunk between the 

 limbs, the muscle-plate (Fig. 11, m.p.) has passed half way down the 

 body wall, lying close outside the cavity. It now consists almost 

 entirely of elongated spindle-cells. In the region of tho limbs 



