204 THE FOURTH DAY. [CHAP. 



The mesoblastic somites, which by the continued 

 differentiation of the axial mesoblast at the tail end of 

 the embryo have increased in number from thirty to 

 forty, undergo during this day changes of great import- 

 ance. Since these changes are intimately connected 

 with the subsequent development of the vertebral 

 column, it will perhaps be more convenient to describe 

 briefly here the whole series of events through which 

 the somites become converted into the permanent 

 structures to which they give rise, though many of the 

 changes do not take j)lace till a much later date than 

 the fourth day. 



The separation of the muscle-plates (p. 1 87) left the 

 remainder of each somite as a somewhat triangular 

 mass lying between the neural canal and notochord on 

 the inside, and the muscle-plate and intermediate cell- 

 mass on the outside (Fig. 64). Already on the third day 

 (Fig. 65) the upper angle of this triangle grows upwards, 

 between its muscle-plate and the neural canal, and 

 meeting its fellow in the middle line above, forms a 

 roof of mesoblast over the neural canal, between it and 

 the superficial epiblast. At about the same time, the 

 inner and lower angle of the triangle grows inwards to- 

 wards the notochord, and passing both below it (betAveen 

 it and the aorta) and above it (between it and the 

 neural canal), meets a similar growth from its fellow 

 somite of the other side, and thus completely invests 

 the notochord with a coat of mesoblast, which, as seen in 

 Fig. 68, is at first much thicker on the under than on 

 the upper side. 



Both neural canal and notochord are thus furnished 

 from neck to tail with a complete investment of meso- 



