THIRD PERIOD OF DEVELOPMENT. 97 



lower, especially in the neighbourhood of the meso bias- 

 tic somites, and therefore require a more important 

 surface extension of the cell plate which is composed 

 of them. 



On the other side, we see from the sections (Figs. 87, 

 89) that the yolk corpuscles in the mesoblastic somite 

 cells are more quickly used up than in the remaining 

 part formed of the primary hypoblast. We can there- 

 fore conclude a preponderating energy of growth in 

 the neighbourhood of the mesoblastic somites. 



The energies of growth and their differences in the 

 epiblast can in the same way be concluded from the 

 condition of the yolk granules in the neural plate 

 and in the rest of the epiblast. If we for instance 

 consider the series of sections (Figs. 86-92, on Table 

 VIII.) of an embryo of 4-5 mesoblastic somites, it 

 must occur to us that the neural plate contains yolk 

 granules far more richly than the rest of the epiblast, 

 so that the cells of the neural plate show more simi- 

 larity with the hypoblast cells than with the more 

 nearly related epiblast cells, and this gives us a 

 deeper insight into the mechanical part of the pro- 

 cesses which have taken place. We shall indeed 

 explain the separation of the neural plate from the 



H 



