350 F M. BALFOUR. 



splanchnic layer is completely separated from the skeleton- 

 forming cells, and at the same time, having united below as 

 well as above with the outer (somatic) layer of the two layers 

 of which the protovertebrae are formed, the two together 

 form an independent mass (fig. 12, mp), simi\a.r in appearance 

 and in every way homologous with the muscle-plate of 

 Birds. 



On the inner side of this, which we may now call the 

 muscle-plate, is seen the bundle of earlier-developed muscles 

 (fig. 12, mp') which I spoke of before. 



The section represented in fig. 12 is from a very consider- 

 ably later embryo than that represented in fig. 11, so that 

 the skeleton-forming cells, few in number in the earlier 

 section, have become very numerous in the later one, and 

 have grown up above the neural canal, and also below the 

 notochord, between the digestive canal and the aorta. They 

 have, moreover, changed their character ; they were round 

 before, now they have become stellate. As to their further 

 history, I will only say that the layer of them immediately 

 around the notochord and neural canal forms the cartilaginous 

 centra and arches of the vertebrae, and that the remaining 

 portion of them, which becomes much more insignificant in 

 size as compared with the muscles, forms the connective tissue 

 of the skeleton and of the parts around and between the 

 muscles. 



A muscle-plate itself is at this stage (shown in fig. 12) 

 composed of an inner and an outer layer of epithelium 

 (splanchnic and somatic) united at the upper and lower ends 

 of the plate, and on the inner of the two lies the more 

 developed mass of muscles before spoken of (mp'). 



Each of these plates now grows both upwards and down- 

 wards; and at the same time connective-tissue cells appear 

 between the plates and epidermis; but from where they 

 come I do not know for certain; very probably they are 

 derived from the somatic layer of the muscle-plate. 



While the muscle-plates continue to grow both upwards and 

 downwards, the cells of which they are composed commence 

 to become elongated and soon acquire an unmistakably 

 muscular character (PI. XV, fig. lS,m p). 



Before this has occurred the inner mass of muscles has also 

 undergone further development and become a large and 

 conspicuous band of muscles close to the notochord (fig. 

 13, in p'). 



At the same time that the muscle-plates acquire the true 

 histological character of muscle, septa of connective tissue 

 grow ill and divide them into ^ number of distinct seg- 



