654 
T. H. MORGAN 
represent the sides of the embryo as His supposed, but that the 
material for almost the entire embryo is laid down in the early 
embryonic shield, and draws together and is carried backward as 
the germ-ring advances. A small contribution from the germ-ring 
probably occurs in the later stages of closing. This interpretation 
is entirely in accord with the evidence furnished by these embryos. 
In several respects these embryos are comparable with those 
that I have described for the frog's egg after centrifuging. The 
defects are, however, due to different factors — here polyspermy, 
there concentration of the yolk.^ The cause of the defect is of 
small importance compared with the interference that defective 
regions exert on the course of development. 
' In my paper on The Centrifuged Frog's Egg, I referred these defects to injuries 
to the yolk field. Here I adopt McClendon's amendment, namely, that the 
yolk hemisphere is not injured, but made unmanageable. To this amendment I 
will add that since in my experiment, the eggs were kept for hours in the 
machine the driving of the resting nuclei into the light hemisphere may be a further 
cause of abnormality. Since the yolk is thereby deprived of nuclei it may be said 
in this rather far-fetched way to be injured. 
