DEVELOPMENT OF ELASMOBRAXCH FISHES. 115 



of the protovertebra3 the primitive position of the split which 

 separated their somatic and splanchnic layers becomes obscured, 

 but when on the third day the muscle-plates are formed they 

 are found to be constituted of tiuo 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 Darwinian principles. The central cavity of the 

 muscle-plate is, in fact, a remnant of vertebral extension of the 

 body-cavity, and is the same cavity as that found in the muscle- 

 plates of Elasmobranchs. 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 protovertebrss in- 

 ternal to the muscle-plates is very large in Birds, and is the 

 equivalent of that portion of the protovertebrse which in Elas- 

 mobranchs is split off to form the vertebral bodies^ (PL x. fig. G, 7, 

 8, Vr). Thus, though the history of the development of the 

 mesoblast is not precisely the same for Birds as for Elasmo- 

 branchs, yet the differences betw^een 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 

 development 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 (protovertebrse) of the plates 

 soon become separated from the parietal, and form an inde- 

 pendent mass of cells constituted of two layers, which w^ere 

 originally continuous with the somatic and splanchnic layers 

 of the parietal plates. The outer or somatic layer of the 



^ Dr Gotte, Entwickhmgsgeschichte der Unke, p. 534, gives a difl'erent 

 account of the development of the protovertebrffi from that in the text. He 

 states that the muscle-plates do not give rise to the main dorso-lateral muscles, 

 but only to some superficial ventral muscles, while the dorso-lateral muscles are 

 according to him formed from part of the kernel of the proto vertebrae 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, Enhokhhmgsge>schichte, 1876, 



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