106 THE PROTOVEKTEBR.E. 



Before the end of stage H the inner or splanchnic wall of 

 the protovertebra loses its simple constitution, owing to the 

 middle part of it, opposite the dorsal two-thirds of the notochord, 

 undergoing peculiar changes. These changes are indicated in 

 transverse sections (PL x. fig. 5 and G mp), by the cells in the 

 part we are speaking of acquiring a peculiar angular appearance, 

 and becoming one or two dee]3 ; and the meaning of the changes 

 is at once shewn by longitudinal horizontal sections. These prove 

 (PL XI. 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 protover- 

 tebra, and in the present stage, or at any rate in stage I, 

 acquires a peculiar granulation, which clearly foreshadows 

 transverse striation (PL xi. fig. 11 — 18). 



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. 76. 



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 

 remaining cells of the protovertebrse. 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 protovertebrse 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. (PL X. fig. 6 and 8). While this is taking place, part of the 

 splanchnic layer of each protovertebra, immediately below the 



