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furnished by teratology. I shall try to show that an alternative view 
is tenable that will explain the facts as well. 
Roux’s best evidence is based on two observations of the frog-egg. 
He found a few embryos in which the nervous system and notochord 
were divided into two parts meeting in the head and tail and with 
a large open blastopore between. Evidently he says the Anlage of the 
nervous system kept apart by the large blastopore has developed in 
situ producing half the nervous system on one side the blastopore and 
half on the other. 
Roux also found that injury to the gastrula gave localized results 
pointing to the same conclusion. If the injury be made to one side 
along the black-white line a corresponding injury will be found on the 
same side of the central nervous system. The evidence seems at first 
conclusive particularly the latter experiment. 
| In a contribution, in press at present, I have described a method 
to produce artificially the open gastrula form. In the same paper I 
have also attempted to show that in the normal egg the dorsal lip of 
the blastepore moves over the yolk through about 120 degrees, how iu 
the embryo produced with open yolk-mass the dorsal lip has remained 
fixed while the lateral lips have slowly extended around the black- 
‘white zone. In other words, by preventing the overgrowth of the 
dorsal lip in the medium line the material has split into two parts 
each moving around the egg in the equatorial line. Such material 
destined to form the nervous system and notochord still produces 
these structures. 
As to Roux’s second experiment one must doubt either the validy 
of it or its interpretation. It seems to me from a study of the normal 
embryo extremely doubtful that the results of such an injury could 
ever pass through such a considerable area to reach ultimately the 
medium line and produce there the defect in the nervous system. The 
experiment needs careful repetition. 
HERTwIG bases his conclusion on the evidence furnished by similar 
embryos to those described by Roux and from the evidence of other 
embryos in which a smaller amount of yolk exposure is present. Also 
on the teretological embryos found in other groups. It is a theory 
of concrescence in a modified sense in as much as only a comparatively 
small portion of the germ-ring is supposed to come together to form 
the embryo. I should differ from Hertwia first as to the time at 
which a portion of the germ-ring is converted into embryonic tissue 
and also as to the method by which this takes place. 
Herrwic believes that the passage of the material of the ring 
