240 NIKOLAS KLETNENBERG. 



appearance, in the first place having lost their vibratile cilia. 

 This alteration in the cells of the furrow takes place bit by 

 bit; in the same embryo, both behind and in front, the cells 

 are found in their characteristic form, and show a lively 

 vibratile movement. I thought that the cells might perhaps 

 transform themselves at a certain time to take part in the 

 production of new ones, and then return to their preceding 

 state, but I have not been able to obtain proofs of this. It 

 is certain that they have no relation with the mesoderm 

 cells, Avhich are found here for the first time interposed 

 between the ectoderm and the medullary plate (fig. 26 a, 

 mes)i because these are derived from the somatic lamina, 

 which, beginning in front, forces itself from the two sides 

 towards the median line, and then backwards, separating the 

 rudiment of the ventral medulla from contact with the 

 ectoderm. 



The groove in the medullary plate, sometimes very deep, 

 which divided the two elevations has now disappeared, or 

 become reduced to a very small impression. The edges of 

 the furrow do not become united, but, on the contrary, the 

 fossa becomes wider and shallower before vanishing, in con- 

 quence of the increase of the medullary cells placed above 

 its floor. The cells which occupy the middle portion of the 

 plate are larger, consist of clearer protoplasm, and have more 

 precise limits than those placed in the lateral portions, from 

 which, however, they are not in any way separated. In the 

 following section (26, h), the nervous plate has changed its 

 form a little, its sides are thickened and form two elevations 

 on the dorsal surface, between which is found a wide and 

 pretty deep furrow. The internal structure also shews some 

 alterations, the greater part of the large median cells being 

 changed and aggregated with the small ones. The proto- 

 plasm of these is dense, and the nuclei fill nearly the entire 

 body of the cell ; they are placed so close together that a 

 high power and great attention are necessary to make out 

 their boundaries; signs of division are frequent. A meso- 

 dermic sheath everywhere surrounds the medullary plate. 



Further in front (fig. 26 c) the rudiment of the nervous 

 chain presents a new form. Till now its lateral wings 

 were elongated, and ended in very sharp points; now they 

 are rounded in such a Avay that their section is kidney 

 shaped. The histological changes met with here are more 

 important. In the dorsal side appear two small, clear-look- 

 ing, finely granular spots, which stain feebly with hsema- 

 toxylin. They have not distinct limits, but lose themselves 

 in the surrounding cells, whose outlines become little by 



