( 444 ) 
rather cilindrical, and divide parallel to the surface of the egg, so 
that one of the daughtercells travels inward. 
At a somewhat later stage of development, when the blastoderm 
has spread further over the surface of the yolk, and the embryonic 
shield is to be seen clearly, a median section gives the following 
view: the invagination of the blastoderm is sharply defined. At the 
hind end of the embryonic shield the cells of the superficial layer, 
which everywhere else are flat and separated by a sharp line from 
the periblast, are cilindrical with the long axe at right angles to 
the surface of the egg, and send a tongue of cells inward between 
the invaginated layer of the blastoderm and the periblast. This is to 
be concluded from the direction of cell division in the projecting 
layer of cells. This tongue of cells consists of loosely packed cells, 
reaches inward almost as far as the invaginated layer of the blasto- 
derm and is distinctly separated from it. 
The thickened part of the superficial layer SuMNER called „prosto- 
mal thickening”. Beneath it are to be seen many nuclei in the peri- 
blast and often one gets the impression as if cells from the periblast 
partake of the process and travel inward with the other cells of the 
prostomal thickening. However I could not state it with a sufficient 
amount of certainty. On cross sections (parallel to the blastoderm- 
ring) the inward proliferation of the superficial layer may also be 
sharply separated from the other blastodermcells. The cells of the 
invaginated layer of the blastoderm form in the median line the 
chorda. The sideparts become the mesodermic plates. The proliferation 
of the superficial layer (with cells from the periblast 7) form the 
gut-entoderm (some cells are separated in the course of development, 
and seem to form primary blood-corpuscles. At least, they do not 
take a part in the forming of any particular layer). 
At the closure of the blastopore the superficial layer forms an 
invagination and in this manner KuprreR’s vesicle is formed, just as 
SUMNER described it in his paper. !) 
In different Muraenoïd spp. however the forming of KuPrrER’s 
vesicle does not take place in the same way. In some spp. (viz. 
Mur. N°. 2) the invagination of the , Deckschicht” (or the overgrowing 
of the prostomal thickening by the tail-knob) begins some time 
before the closure of the blastopore, in other spp. (viz. Mur. No. 1, 
NO. 3) it begins much later, as the blastopore is nearly closed and 
*) RaFraELE too saw a transitory communication of KuprreEr’s vesicle with the 
exterior in the Muraenoids. 
