No. 2.] VERTEBRATE CEPHALOGENESIS. 235 



cells, occurs concomitantly with the development of the meso- 

 dcrmic somites and the peripheral sense organs. 



This resting stage into which the cells of the medullary plate 

 as a whole pass, as soon as the plate has been fully formed, illus- 

 trates the retardation of the development of the structure, and 

 consequently function, of the apparatus, and this fully serves to 

 explain why the superficial sensory organs and their pigment 

 bodies remain undeveloped until the nervous system is entirely 

 inclosed. For, although the medullary plate enters on a resting 

 phase, the remaining organs of the body continue their develop- 

 ment, especially the mesodermic structures, which by rapid 

 growth inclose the nervous plate long before the resting phase 

 has passed, and the differentiation of the permanent fibres and 

 ganglion cells of the nervous cord begins. 



H. EacJi one of the pigment bodies is connected with, forms a 

 deposit in, an amoeboid cell. All these cells retain their amceboid 

 nature thro7ighout life, the pigment cells of the eye-spot not 

 excluded. 



The pigment-bearing cells are relatively large, being in the 

 majority of cases equal to the middle size ganglion cells. The 

 pigment makes its appearance in the cells of the growing larva 

 in the form of particles of melanin, which may or may not fuse 

 into a single mass. The particles are related in some unknown 

 way to the protoplasmic structure of the cell, and are controlled 

 by the cell, massed together in a more or less irregular lump in 

 the contracted condition, or spread out in the form of threads, 

 sheets, rows of particles, etc., in the expanded condition, of the 

 cell. 



The pigment cells arise bilaterally near the centre of the 

 somites, and multiply in such a way that they take in more or 

 less of the segment longitudinally. In the contracted condition 

 the pigment frequently appears in the form of crescentic bodies, 

 hemispherical cups, either simple or with rays streaming out 

 from the periphery. 



The nucleus of the cell is frequently visible, though the cell 

 wall is usually hidden by the pigment. 



The nucleus is eccentric in position and in carmine stains, 

 it colors, with its cell, much like the ganglion cells with which 

 it is associated. 



