114 THE DEVELOPMENT OF THE CHICK 



was made, the concurrent testimony of which was perfectly 

 conclusive. 



The somite formed in front of the first cleft is thus the first 

 in position of the definitive series and the remainder arise in suc- 

 cession behind it. The formation of the somites therefore follows 

 the usual law of antero-posterior differentiation. 



There is always a stretch of unsegmented paraxial mesoblast 

 between the last somite and the anterior end of the primitive 

 streak. 



The first four somites belong to the head, and enter into the 

 composition of the occipital region. The more anterior part of 

 the mesoblast of the head never becomes segmented in the chick. 

 In the anamniote vertebrates, segmentation of the mesoblast 

 extends farther forward, and there is a greater number of cephalic 

 somites. This may be taken as evidence that a large part, at 

 least, of the head was primitively segmented like the trunk. 

 As we shall see later, the primitive metamerism of the head is 

 also expressed in other ways: neuromeres, branchiomeres, etc. 



The segmentation of the mesoblast finally extends to the 

 hind end of the tail, new segments being continually cut off 

 from the anterior end of the paraxial mesoblast until it is all used 

 up. This is not complete until the fifth day. The number of 

 somites thus formed is perfectly constant, as is also the fate of 

 the individual somites. 



Primary Structure of the Somites. Each somite is primarily 

 a block of cells arranged in the form of an epithelium around a 

 small central lumen, towards which the inner ends of all the cells 

 converge (Fig. 68 B). The central cavity (myocoele) is, however, 

 filled with ar^ irregularly arranged group of cells, and, though 

 the cavity must be regarded as part of the primitive body-cavity, 

 or ccelome, it has no open communication with it. After the 

 somites are formed they rapidly become thicker so that their 

 lateral boundary becomes very sharply marked; this is not due 

 to a longitudinal constriction external to the paraxial mesoblast, 

 as usually stated. Each somite has six sides, of which five are 

 free, viz., dorsal, ventral, anterior, posterior, and median. The 

 sixth or lateral side is continuous with the nephrotome. 



The Nephrotome, or Intermediate Cell- mass (Middle Plate). 



