591 
of Edinburgh, Session 1885 - 86 . 
The first two furrows in the herring are vertical in direction, and 
at right angles to one another. They are not pushed down to the 
base of the germinal mound, but cease some distance before reach- 
ing the yolk. Four segmentation spheres are thus formed, which 
are imperfectly separated from the lower pole of the ovum. The 
third furrow takes an equatorial direction, and simply completes 
the bases of the first four segmentation spheres. The upper portion 
goes on segmenting, and constitutes the archiblast. The lower pole 
consists of a central mass of yolk, around which is a layer of 
protoplasm, -which constitutes the parablast. With the formation of 
the first equatorial furrow we have a division of the ovum into 
an animal and a vegetative pole, as in the amphibian ovum. 
The archiblast represents the animal pole, and the parablast, with 
its enclosed yolk, the vegetative. The segmentation cavity appears 
between them. The primitive hypoblast is formed from the 
vegetative pole by a budding off of cells from the margin of the 
parablast, as has already been described. Cell formation takes place 
in the parablast of the herring ovum earlier than I have observed it in 
other forms, and two batches of cells are added to the animal pole by 
this means before the pseudo-invagination begins. In the herring 
the archiblast, together with the cells which have been derived 
from the parablast prior to the formation of the segmentation 
cavity, gives rise to the ectoderm. The primitive hypoblast, which 
is almost entirely derived from the vegetative pole, gives rise to the 
mesoderm and notochord, and the true entoderm remains as a single 
row of cells in connection with the parablast. 
If this account should prove to be correct, several interesting 
relations will be established between Teleostei and other forms. 
The primitive hypoblast, as I have here described it, is precisely 
homologous with that of Amphioxus, and both give rise to meso- 
derm, notochord, and entoderm. 
Again the derivatives of the animal and vegetative pole in the 
herring are practically the same as in the frog. In Teleosteans, 
however, the vegetative pole consists at first of one cell instead of 
four, this arrangement being brought about by the greater bulk and 
different arrangement of the yolk. 
Quite recently Dr Ruckert, who has investigated the question 
in Elasmobranchs, has come to very similar conclusions. 
