590 
Proceedings of the Royal Society 
Again, developing the six determinants \zf\ , {yt\ , .... in the 
same way, and rearranging the terms, we have finally 
[xyzt\ = {(ah' -a'b)c" - (ad -a'd)b" + (bd -b'c)a"}t 
- {(ad - a’b)d" - (ad' - a'd)b n + (bd' - b'd)a"}z 
+ {(ad - a'd)d" - (ad' - a'd)d' + (cd' - dd)a"}y 
- {(bd - dd)d" - (bd' - dd)c" + (cd' - dd)b"}x. 
Bat the coefficients of x, y , z , t in \xyz £] were seen on starting to 
he the numerators and the common denominator of the values of 
x, y , 2 in the given set of equations : hence 
- {(bd - dd)d" - (bd' - dd)c" + (cd' - c'd)b"} 
{(ad - a'b)c' - (ad - a'c)b" + (bd - b'd)a") 
y= 
z— 
2. The Formation of the Germinal Layers in Teleostei. 
By George Brook, F.L.S., Lecturer on Comparative 
Embryology in the University of Edinburgh. 
(Abstract.) 
The majority of recent investigations have concluded that the 
thickening of the rim of the blastoderm, and the new layer which 
is derived from it, is brought about by a true invagination of the 
segmented blastoderm. On the other hand, Klein and Kupffer hold 
that the newly formed layer is not derived from the segmented 
blastoderm (archiblast), but is mainly produced from an unsegmented 
portion of the blastoderm (parablast), which is intermediate in 
position between the archiblast and the yolk. 
A little over a year ago I was led to conclude, from a study 
of the development of Tradhinus , that the so-called “ invaginated 
layer,” the primitive hypoblast, was in great part, if not entirely, 
built up from the margin inwards of cells budded off from the 
parablast. More recently I have studied the question in other 
forms, particularly the herring, and am now enabled to bring for- 
ward more important evidence as to the origin and morphological 
significance of the parablast, and also of the part which this layer 
plays in the economy of the embryo. 
