324 DEVELOPMENT OF ELASMOBRANCH FISHES. 



going peculiar changes. These changes are indicated in trans- 

 verse sections (PL 11, figs. 5 and 6 mp'\ by the cells in the part 

 we are speaking of acquiring a peculiar angular appearance, and 

 becoming one or two deep ; and the meaning of the changes is 

 at once shewn by longitudinal horizontal sections. These prove 

 (PI. 12, fig. 10) that the cells in this situation have become elong- 

 ated in a longitudinal direction, and, in fact, form typical spindle- 

 shaped embryonic muscle-cells, each with a large nucleus. Every 

 muscle-cell extends for the whole length of a protovertebra, and 

 in the present stage, or at any rate in stage I, acquires a peculiar 

 granulation, which clearly foreshadows transverse striation (PI. 

 12, figs. 1113). 



Thus by stage H a small portion of the splanchnopleure 

 which forms the inner layer of each protovertebra, becomes 

 differentiated into a distinct band of longitudinal striated muscles; 

 these almost at once become functional, and produce the peculiar 

 serpentine movements of the embryo, spoken of in a previous 

 chapter, p. 291. 



It may be well to say at once that these muscles form but a 

 very small part of the muscles which eventually appear ; which 

 latter are developed at a very much later period from the re- 

 maining cells of the protovertebrae. The band developed at this 

 stage appears to be a special formation, which has arisen through 

 the action of natural selection, to enable the embryo to meet its 

 respiratory requirements, by continually moving about, and so 

 subjecting its body to fresh oxydizing influences ; and as such 

 affords an interesting example of an important structure acquired 

 during and for embryonic life. 



Though the cavities in the protovertebra are at first per 

 fectly continuous with the general body-cavity, of which indeed 

 they merely form a specialized part, yet by the close of stage H 

 they begin to be constricted off from the general body-cavity, 

 and this process is continued rapidly, and completed shortly 

 after stage I, and considerably before the commencement of 

 stage K (PI. n, figs. 6 and 8). While this is taking place, 

 part of the splanchnic layer of each protovertebra, immediately 

 below the muscle-band just described, begins to proliferate, and 

 produce a number of cells, which at once grow in between 

 the muscles and the notochord. These cells are very easily 



