( 445 ) 
the periblast almost entirely overgrown. The first mode of develop- 
ment gives rise to a KUPFFER’s vesicle, bounded on the dorsal side 
with epithelium, on the ventral side with periblast, the second to a 
KurrreR's vesicle almost entirely bounded with epithelium. 
The wall of Kuprrer’s vesicle is continuous with the wall of the 
gut. The dorsal row of cells forms the hypochorda. 
It would lead us too far to describe the later development of the 
gut, the concentration of cells towards the median line, the folding 
and forming of the gut-tube. I will here restrain myself to show, 
how in these Teleosts the secondary entoderm — the gut-entoderm — 
is formed independently of the chorda and the mesodermic plates, 
and how the chorda is differentiated out of the median part of the 
invaginated layer of the blastoderm. For that seems to me to be of 
great value to understand rightly the processes of development that 
take place in the head-part of the embryo. 
In following the development of the head on median sagittal 
sections, as are lying before me in sufficient quantities and of diffe- 
rent stages of development, the observer sees, that in the stage in 
which the central nervous system forms only a solid keel and there 
is as yet nothing to be seen of the optic vesicles, the chorda, until 
the point where in later stages it ends, is composed of flat disk-like 
cells, but then grows thinner and can be traced as two rows of cells 
dorsally of the entoderm to the foremost part of the head, where they 
form a big mass of cells lying in front of the brain and beneath 
the point where the anterior neuroporus is formed. The cells of the 
entoderm can everywhere be distinguished sharply from the pro- 
longation of the chorda, except in the foremost part, where in median 
sections it was difficult to distinguish them from each other. As soon 
as the optic vesicles are formed, the infundibulum develops and pushes 
the mesodermeells away. On median sections the prolongation of the 
chorda is now to be followed until it reaches the infundibulum. In 
front of the infundibulum the anterior mesodermmass is then to be 
seen. The ectoderm grows inward beneath this mass of cells and unites - 
with the entoderm. But the cells of the entoderm not containing 
any particles of yolk here, and being onty distinguishable by their 
position from those of the other layers, I cannot yet fix accurately 
the point where the entoderm and the ectoderm fuse. The ectoderm 
seems always to be sharply separated from the anterior mesodermmass, 
that in later stages forms the sceletogenous tissue of the fore-head. 
In following the further development of the mesoderm of the head 
in median sections, the chorda is seen to be rounded off and to be 
separated from the anterior row of cells; by the growth in length 
