233 NIKOLAS KLEINENBERG. 



continuous with the ectoderm, the cells of which are cylin- 

 drical on the edge of the projection, but on the dorsal surface 

 from their, plates, which appear fusiform in section. But 

 what has lately happened is that for a small space the ecto- 

 derm has become thickened ; it consists here of two sets of 

 cells, while a short time before it w^as everywhere composed 

 of a single layer. The cells of this thickening [gc) are not, 

 however, arranged in distinct layers, but are closely united 

 into a single mass ; it is exactly and clearly limited by the 

 somatic lamina. The unfigured sections of the same series, 

 which are immediately to the right and left of the one de- 

 scribed, show the same characters, with the difference only 

 that the number of cells composing the thickening is smaller; 

 tlie same is observed also in the following sections on each 

 side, although these are very oblique. In the sections still 

 more to the sides the thickening disappears altogether, and 

 the ectoderm becomes again unicellular. 



Examining now the head end of a slightly more developed 

 embryo by means of transverse sections, we see in the first 

 (PL X, fig. 20 a), which passes only through the semilunar 

 projection of the cephalic zoonite, a group of small cells 

 {gc), rather thinned in the middle, completely separated from 

 the mesoderm, which is here in an abnormally retarded state 

 of development, not being yet split in the median line, and 

 beginning to separate itself from the superficial layer of 

 ectoderm. The section immediately following this (fig. 20 h) 

 shows how these cells pass directly into a very conspicuou 

 enlargement of the ectoderm in the median dorsal line, which 

 here is composed of as many as four layers of cells. In the 

 third (fig. 20 c) the thickening of the ectoderm, although 

 diminished in the median line, is increased at the sides, 

 where it descends for a good distance towards the ventral 

 surface, becoming gradually thinner, and at last unicellular. 

 This is shown best in the left side of the figure, the section 

 being a little oblique. In the fourth section the ectodermic 

 thickening may be still seen, though it is much diminished. 

 In the following sections it exists no longer. 



The series shown in figs. 19 a, h, c, d, is taken from a still 

 more developed embryo. In the first section (fig. 19 a) the 

 thickening of the ectoderm embraces, in the form of a half 

 circle, the superior convex part of the cavity of the head {cc), 

 from which, however, it is separated by the thin membranous 

 somatic lamina (Iso). On the external surface of tlie thick- 

 ening a single layer of fiat pavement-cells (ec) is separated 

 from the internal mass, composed of roundish cells with re- 

 latively very large nuclei; in other words, the rudiment of 



