STAGES G TO K. GENERAL CONSIDERATIONS. 333 



on the third day the muscle-plates are formed they are found to 

 be constituted of two layers, an inner and an outer, which enclose 

 between them a central cavity. This remarkable fact, which has 

 not received much attention, though noticeable in most figures, 

 receives a simple explanation as a surviving rudiment on Dar- 

 winian principles. The central cavity of the muscle-plate is, in 

 fact, a remnant of the vertebral extension of the body-cavity, and 

 is the same cavity as that found in the muscle-plates of Elasmo- 

 branchs. The two layers of the muscle-plate also correspond 

 with the two layers present in Elasmobranchs, the one belonging 

 to the somatic, the other to the splanchnic layer of mesoblast. 

 The remainder of the protovertebrae internal to the muscle-plates 

 is very large in Birds, and is the equivalent of that portion of the 

 protovertebrae which in Elasmobranchs is split off to form the 

 vertebral bodies 1 (PI. 11, figs. 6, 7, 8, Vr). Thus, though the 

 history of the development of the mesoblast is not precisely the 

 same for Birds as for Elasmobranchs, yet the differences between 

 the two groups are of such a character as to prove in a striking 

 manner that the Avian development is a derivation from a more 

 primary form, like that of the Elasmobranchs. 



According to the statements of Bambeke and Gotte, the 

 Amphibians present rather remarkable peculiarities in the develop- 

 ment of their muscular system. Each side-plate of mesoblast is 

 divided into a somatic and a splanchnic layer, continuous 

 throughout the vertebral and parietal portions of the plate. The 

 vertebral portions (protovertebrae) of the plates soon become 

 separated from the parietal, and form an independent mass of 

 cells constituted of two layers, which were originally continuous 

 with the somatic and splanchnic layers of the parietal plates. 

 The outer or somatic layer of the vertebral plates is formed of a 

 single row of cells, but the inner or splanchnic layer is made up 

 of a central kernel of cells and an inner single layer. This 

 central kernel is the first portion of the vertebral body to undergo 



1 Dr Gotte, Enlwicklungsgeschichte der Unke, p. 534, gives a different account of 

 the development of the protovertebrse from that in the text. He states that the 

 muscle-plates do not give rise to the main dorso-lateral muscles, hut only to some 

 superficial ventral muscles, while the dorso-lateral muscles are according to him formed 

 from part of the kernel of the protovertebrse internal to the muscle-plates. The 

 account given in the text is the result of my own investigations, and accords precisely 

 with the recent statements of Professor Kolliker, Enlwicklungsgeschichte, 1876. 



