78 LEONARD W. WILLIAMS 



the process is beginning with somites of intermediate or anterior 

 positions in the series which show successively later stages." 

 Kollmann expresses the same idea more specifically (p. 43 and 

 44) :- ''A myotome from the posterior part of the trunk will be 

 represented because its condition immediately follows that of the 

 mj^otome of the embryo of two weeks, which is not the case with 

 the myotomes of the anterior part of the trunk. There, in cor- 

 respondence with the more advanced development of the anterior 

 part of the body, the myotomes are considerably more advanced 

 than those of the posterior part of the trunk. One can therefore 

 make out various stages of development of this muscle organ in 

 a single embryo." 



Our attention is at once arrested by the great variation in the 

 size of the somites. There is, from in front backward to the thir- 

 tieth, a gradual increase in the size of the somites; beyond this, 

 that is, in the caudal region, the somites gradually become smaller. 

 The fifth somite is apparently an exception for it is nearly as 

 large as the fifteenth. 



A distinct core appears first in the ninth or tenth somite and 

 each succeeding somite as far back as the thirtieth or thirty-fifth 

 segment has a larger core. The greater size of the more posterior 

 somites is due largely to the fact that the somitic plate from which 

 they are formed is thicker posteriorly. The structures of the 

 anterior part of the embryo grow more rapidly than those of its 

 posterior part and consequently each somite is always larger than 

 those behind it. In other words, the anterior somites, which at 

 the start are smaller and more primitive than the posterior somites, 

 because of their more rapid growth always keep in advance of the 

 latter. Thus we see that the second somite (fig. 5) of an embryo 

 of twelve segments is larger, as well as more advanced, than the fif- 



- "Es 8oll hior zunjiclist cin Myotoin aus dcin liintei'cn Rumpfabschnitt dieses 

 Embryo geschildert wcrden, weil sich dessen Verhallen unmittclbar an dasjenige 

 des zwei Wochen alten Embryo anschliesst, was mit den Myotomen im Vorderrumpf 

 nicht der Fall ist. Dort sind sie in Uebereinstimmung mit der ganzen vorge- 

 schrittenen Ausbildung dcs Vorderkorpcrs lietraclitlich dcnen dcs Hinterrumpfes 

 voraus. Man kann also verschiedene Entwickelungsstufen dieses Muskelorganes 

 kennen lernen an einem iind demselben Embryo." 



