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distance they fill in the floor of the ventral wall of the alimentary 
canal. Soon after this period the gut becomes shut off from the yolk 
by ingrowths from the sides. This was described in my preliminary 
paper'), but that account is now hardly accurate enough for my 
purpose. | 
As previously stated, the gut is in a sense completely shut off 
from the yolk from a very early period of the development, or to 
speak more exactly, the larval gut has, as its ventral wall, the mero- 
cytic layer on the top?) of the yolk-sac. At the critical period, which 
in Lepidosteus arises when the embryo is about 9 mm in length, the 
formation of the embryonic gut, as opposed to the larval gut, has well 
set in. This permanent gut is formed, as previously recorded, by in- 
growths from the sides, and the ventral wall, or merocytic layer, takes 
no share in its formation. The gut, thus differentiated, soon closes in 
to form a tube, except at one point, which is, at first, rather large, 
but which, by and by, becomes much more restricted in extent. Here 
there is a wide passage from the embryonic gut leading on to the 
yolk-sac, or, to put it differently, here the ventral wall is formed by 
merocytes. 
In a sense it is difficult to speak of an internal yolk-sac in Le- 
pidosteus*), for the yolk-sac, though it does get seized upon by the 
embryo, is annexed in a fashion different from that adopted in Scyl- 
lium. The wide passage or yolk-bay above described corresponds mor- 
phologically to the duct leading in Scyllium from the gut to the yolk- 
store, at first to the vitelline duct, then, when it becomes narrowed, 
to the passage leading from the gut to the internal yolk-sac. 
Prior to the critical period the embryo was nourished by the yolk 
contained in its cells, and perhaps to some extent by the yolk pre- 
pared by merocytes for absorption by the blood-vessels of the sac. 
It is not until the critical period is reached, that yolk is found in the 
gut of the embryo. In embryos of 9 mm, in which among other things 
the degeneration of the transient ganglion-cells is just beginning, and 
in which the formation of the permanent central canal of the spinal 
cord is just initiated, this is the case. — There is yolk in the 
gut. This, however, is not: by any means all. It was rather antici- 
pated that the course of events would be found to correspond to that 
1) loc. cit. p. 114. 
2) It is not to be forgotten that other parts of this ventral wall of the 
gut occur in other parts of the yolk-sac, i. e. around its circumference. 
3) What Hans Vırcmow denominates “Dotter-bucht” or yolk-bay would 
better describe the actual conditions. 
