•CYTOLOGICAL STUDIES OF CEXTRIFUGED EGGS 
653 
ganized cells, and these have no doubt been the cause of the lack 
of development on this side. The next embryo ^fig. Q) is also 
defective on one side, at least as far as concerns the mesoderm. A 
small lump at the edge of the overgro^yn part represents, no doubt, 
the defective material of the missing half. In fig. R the posterior 
end of the embryo has failed to advance over the yolk at the same 
rate as the germ ring. In consequence the embyo has remained 
short and thick. A later stage of this embryo, when the germ ring 
has nearly closed, is shown in fig. The elongated blastopore 
occupies the region normally taken up by the elongated embryo. 
It is not without interest to find an elongated slit, showing as it 
does that the anterior lip of the blastopore, after reaching a certain 
point, fails to advance further. In fig. S a defective embryo is 
shown in which the head is .twisted and the posterior end spread 
out widely. Similarly the embryo in fig. T is defective in several 
respects. Finally fig. U shows an embryo in which apparently 
th^- nerve cord and notochord have developed side by side instead 
of above and below. 
These embryos, after drawing, were set aside to be studied by 
means of serial sections, but much to my regret the bottles con- 
taining them were lost in transportaion. The main features at 
least are apparent from the surface views. 
These embryos have, I think, some general interest in connection 
with the question of the mode of formation of the embryo, 
whether by elongation orconcentration, or by concrescence. I have 
looked in vain among all the embryos with wide open blastopore 
for evidence of differentiation of the material of the gerni-ring, 
which would be expected on the concrescence theory. I have never 
found any evidence of such development. The nearest approach 
to such a condition is shown in the half embr^'o of fig. P, but here 
the mode of formation is clearly of another sort. Owing to injur}- 
of one side of the blastoderm the embr3'onic knob has extended 
backward along the middle and one side, while the material of the 
other side has been largely held back in consequence of the defec- 
tive condition of that side. These embryos support the conclu- 
sion that I reached by experimental methods in 1895 which Kopsch 
confirmed later ; namely that in the teleost the germ-ring does not 
