2 14 Vertebrate Embryology 



changes that take place in the mesoblastic 

 somites. 



The formation of the mesoblastic somites 

 has already been described, and it will be 

 remembered that, at the end of the second 

 day, they were more or less triangular masses 

 of tissue, frequently with a small central 

 cavity, probably a continuation of the body- 

 cavity. Each somite now increases in depth, 

 and its cavity, the fnyocoel, shifts its position 

 until it lies in the upper part of the somite, 

 instead of near the centre. Then the upper 

 part of the somite, with the myocoel, separates 

 from the lower part, and forms what is known 

 as the muscle plate. The muscle plate consists 

 of closely packed cells, while the lower part of 

 the somite is made up of loosely arranged 

 cells of the stellate form so characteristic of 

 undifferentiated mesoblast (Fig. 48, vies). The 

 muscle plates are, at first, nearly horizontal, 

 with their inner ends slightly dorsal to their 

 outer ends, but they become more and more 

 steeply inclined until, at the end of the third 

 day, they are almost vertical (Fig. 69, MM^. 



The cells of the ventral walls of the muscle 

 plates become converted into bands of longi- 

 tudinal muscle fibres, which bands remain di- 



