VI.] THE PHOTO VERTEBRAE. 151 



11. The protovertebra?, which by the continued differ- 

 entiation 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 importance. 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 protovertebraa become converted into the per- 

 manent structures to which they give rise, though many of 

 the changes do not take place till a much later date than 

 the fourth day. 



The separation of the muscle-plates (Chap. v. § 23) left 

 the remainder of each protovertebra as a somewhat tri- 

 angular mass lying between the neural canal and notochord 

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

 on the outside (Fig. 44). Already on the third day 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 towards the notochord, and passing 

 both below it (between it and the aorta) and above it (be- 

 tween it and the neural canal), meets a similar growth from 

 its fellow protovertebra of the other side, and thus com- 

 pletely invests the notochord with a coat of mesoblast, 

 w r hich, as seen in Fig. 47, is at first much thicker on the 

 under than on the upper side. 



While the inner portion of each protovertebra is thus 

 extending inwards around both notochord and neural canal, 

 the remaining outer portion is undergoing a remarkable 

 change. It becomes divided into an anterior or prceaxial, 

 and a posterior or postaxial segment. The anterior, which is 

 the larger and more transparent of the two, is the rudiment 

 of the spinal ganglion and nerve, while the posterior, which 

 remains more particularly connected with the extensions 

 round the neural canal and notochord, goes to form part of 

 the permanent vertebra. 



In this way, each protovertebra, having given rise to a 

 muscle-plate, is further divided into a ganglionic rudiment, 

 and into a mass which we may speak of as a "primary' 



