18() THE DEVELOPMENT OF THE TADPOLE. 



distinct sets — the one voluntary, the other invokmtary ; the one 

 seen in all muscles employed in the movement of the various 

 organs of the body which are under the direction of the will ; 

 whilst the other set is to be looked for in such structures as the 

 arteries, intestines, etc., whose muscular powers are not under the 

 control of the will. 



The voluntary muscular system has from the very first a most 

 intimate connection with the brain, as seen in Fig. 2, PI. XVII., 

 where the connection between the two may be traced, the muscle- 

 plates running down on either side of the brain in wavy lines and 

 ultimately losing themselves in undifferentiated yelk-tissue. 

 Before taking on this wavy character, these muscle-plates consist 

 of rounded cells, and may be traced as a direct continuation of 

 the mesoblast all round the egg several days before it leaves the 

 capsule, at the sides consisting of only one layer of cells, but in 

 the front part, where the greatest amount of development is going 

 on, they form a layer of six or more rows. Each cell can be 

 traced as a distinct round body, but soon arranging themselves in 

 lines they commence to elongate much faster than the whole egg 

 grows in the same direction, and are thus thrown into wavy lines 

 of cells, the wavy character being first observed nearest the 

 notochord and undifferentiated yelk. 



Although I have not been able to trace the exact steps in the 

 process, I am inclined to think that many of these cells join 

 together to form one long cell, because in sections whose life- 

 history is separated by only two days, one contains chains of 

 round cells touching each other, whilst the other contains distinct 

 waves of elongated cells, which run from the front and embrace 

 the most important portions of the embryo. These muscular 

 lines becoming less wavy lengthen out and envelope the whole 

 brain and notochord at a point nearly as far as the blunt termina- 

 tion of the latter. Divisions between the wavy parts are seen at 

 an early stage, but they become more pronounced and numerous 

 till the full number have been attained, which does not take place 

 until the creature has a strong and fully-grown tail. 



It will be remembered that voluntary and involuntary muscles 

 can be readily distinguished by the striped character of the one 

 and the unstriped character of the other. This is, however, by no 



