1.36 THE THIRD DAY. [CHAP. 



upper and outer border of the protovertebra become separated 

 from the rest of the cortex, the columnar cells of the latter 

 at the same time losing their distinctive characters and 

 ceasing to be distinguishable from the central cells. As a 

 consequence of this the whole protovertebra, while thus 

 increasing in breadth out of proportion to its height, becomes 

 split up into two portions which lie one above the other. 

 Of these the upper one, which is from the first the most 

 flattened and longest, follows the curvature of the body-wall 

 and thus from being nearly horizontal comes to slope at a 

 considerable angle. It now receives the name of muscle- plate, 

 Fig. 44, m.p. Of its subsequent changes we shall have to speak 

 in a succeeding chapter. 



The remaining portion of the original protovertebra is 

 still called protovertebra and begins to extend inwards over 

 the neural canal above and towards the notochord below. 



24. Meanwhile the breadth or rather depth of the trunk 

 is also being increased by the development of mesoblastic 

 cells between the notochord and hypoblast. 



In a transverse section of a 45 hours' embryo a consider- 

 able mass of cells may be seen collected between the pro- 

 tovertebra and the point where the divergence into somato- 

 pleure and splanchnopleure begins (Fig. 20 just below W.d., 

 also Fig. 41, the diagrammatically shaded part lying between 

 p.v. and g.e). This mass of cells, which we may speak of as 

 the intermediate cell-mass, now passes without any very sharp 

 line of demarcation into the protovertebra itself; and as the 

 folding in of the side wall progresses, increases in size and 

 grows in between the notochord and the hypoblast, but does 

 not accumulate to a sufficient extent to separate them 

 widely until the end of the third or beginning of the fourth 

 day. 



The fusion between the intermediate cell-mass and the 

 outer and under portions of the altered pro to vertebras 

 becomes so complete on the third day that it is almost 

 impossible to say which of the cells in the immediate 

 neighbourhood of the notochord are derived from the proto- 

 vertebrse and which from the intermediate cell-mass. It 

 seems probable however that the cells which form the 

 immediate investment of the notochord really belong to the 

 protovertebra}. 



