238 NIKOLAS KLEINENBERG. 



When the mesoderm of the germinal streak has fused in 

 the median line, the ectoderm is still divided into two 

 lateral sheets by the narrow band of ciliated cells, which runs 

 along the whole ventral surface. The cells of this band, 

 besides being covered with vibratile cilia, are clearly distin- 

 guished from the rest of the ectoderm by their transparent 

 appearance, their granular protoplasm beingreplaced toa great 

 extent by a very transparent substance, and reduced to a fine 

 network radiating from the large nucleus, and a condensed 

 layer on the side which bears the cilia. These cells at first 

 project and form a low crest, but afterwards become raised 

 at the sides, so that a longitudinal furrow appears between 

 them, which I shall call the ventral furrow. 



At this time the first trace of the developing nervous cord 

 appears as two thickenings of the ectoderm, immediately on 

 each side of the ventral furrow. These are still so little 

 raised that it is impossible to detect them by looking at the 

 embryo from the front ; but transverse sections show thatone, 

 two, or three cells have been newly formed in the ectoderm, 

 and are placed partly among and partly beneath the pre- 

 existing cells. There can be no doubt as to their origin, for 

 they are perfectly separated from the mesoderm, while 

 they are united into a single mass with the ectoderm, from 

 which many cells show the most evident signs of being in a 

 state of division. Then, at each side of the groove, one 

 of the deep cells of the ectoderm assumes an appearance 

 rather different from the rest, becoming darker, in conse- 

 quence of the condensation of its protoplasm ; it is still 

 further marked by its broader and more distinct outline, an 

 outline which the other ectodermic cells do not possess. 

 The two cells thus distinguished, separated from one another 

 by the epithelium of the furrow, are the first stage of the 

 ventral cord. Sometimes it appears that two or three 

 ectoderm cells become changed at the same time, but in 

 general the process begins in a single cell. This, however, 

 divides without delay, and then two well-defined groups of 

 two or three cells each are seen in the transverse section, on 

 either side of the ciliated cells (PI. IX, fig, 14 n). In this 

 way are developed along the ventral furrow two cords, broad 

 and clearly defined in front, becoming thinned away behind, 

 where they finally blend with the primitive ectoderm. 

 Here the process of division in the ectoderm cells continues, 

 and hence the prolongation of the cords is principally 

 eff'ected by the addition of freshly separated cells, Avhile the 

 increase of their thickness is produced by means of the cells 

 already transformed ; it is not, however, impossible that^ in 



