Ch. XIV] 



ORGANS FROM THE MESODERM 



149 



outer side of each somite by tlie entire somatic layer. Along 

 the ventral and median boundaries of the somites a layer 

 having a loose epithelial character (mesenchyme) is also to be 

 seen. Thus the central mass whicli is to develop into the 

 myotome lies on the median side of the coelom, and is wholly 

 surrounded by an epithelial layer. Frontal sections show that 

 this layer can also be traced inward for some distance between 

 successive somites over both their anterior and posterior sur- 

 faces (Fig. 44). 



"Not merely is mesenchyme produced by the thin peripheral 

 layer of the somites, but in anterior regions considerable por- 

 tions of the kernels of the somites also undergo a metamor- 

 phosis in this direction. Thus, if I be not mistaken, a somite 

 immediately in front of somite 1 has been wholly converted 

 into mesenchymatic tissue. The kernel of the succeeding so- 

 mite (somite 1) has given rise to a considerable quantity of 

 mesenchyme, and the process has been manifested, though to a 

 less degree, even in succeeding somites." ^ 



At the time when fourteen pairs of somites are present 2 

 the cells of the more 

 anterior somites have 

 begun to differentiate 

 into muscle-fibres. The 

 cells of each somite elon- 

 gate in the antero-pos- 

 terior direction a n d 

 become cylindrical in 

 shape, and each extends 

 the whole length of its 

 somite (Fig. 44, B). 

 Each cylindrical cell has 

 at first but a single nu- 

 cleus. Around the wall 

 of the cell a layer of fine 



fibrilke appears. The original nucleus divides and re-divides 

 into man}' nuclei, which lie scattered throughout the cell. 



1 Field ('91). 



- Four days after fertilization of the egg, when three pairs of gills have 

 appeared. 



MS 



B 



Fig. 44. — Frontal sections through the anterior 

 end of Boml)inator. (After Gotte.) A. Shows 

 three gill-pouches (G), and mesoderm of 

 arches. B. Shows formation of mesodermic 

 somites (MS). PH. Pharynx. 



