THIRD PERIOD OF DEVELOPMENT. 117 



these flat-celled layers extend more and more the 

 opposition between these and the columnar muscle 

 cells becomes more and more noticeable. The latter 

 preserve their limited extension on the lateral surfaces 

 of the notochord. 



The form of the muscle cells becomes club-like on 

 the transverse section, while their interior ends, 

 which are in contact with the notochord, become 

 considerably smaller in comparison with the exterior 

 ends turned towards the lumen of the mesoblastic 

 somites. Thereby the transverse section of the 

 series of muscle cells assumes a fan-shaped form. On 

 the transverse section are found the cell nuclei in the 

 club-like extremities of these cells. On one section, 

 however, these are not visible in all cells, since, indeed, 

 as we have seen before (Fig. 51), each cell extends 

 through the whole length of the mesoblastic somite ; 

 and in all this extension possesses but one single cell 

 nucleus. 



We will now, in transverse section, examine the 

 anterior outgrowth of the end of the first mesoblastic 

 somite. It can be seen that during the closing of 

 the communicating slit the first mesoblastic somite 

 extends forward into a blunt prolongation (Figs. 93, 



