OTIC AND OPTIC PRIMORDIA IN MAN 225 
not consider all the factors involved and that the results may be 
explained differently. It may be that his median extirpations 
had a much more general effect than he assumed and were in 
fact comparable to the general inhibition experiments with 
anaesthetics. The growing tip of the nervous system was re- 
moved, and this, in terms of Child’s gradient hypothesis, is the 
dominant region of highest metabolism. According to the 
severity of the injury, the development of one or both eyes was 
more or less inhibited. The extirpations of lateral areas would 
be more convincing if there had been any attempt to map out 
the morphologically differentiated optic areas and remove them. 
Even then the regulatory restitution of an entire optic vesicle 
from a fragment of the primordium intrudes itself. It might 
prove possible to make small definitely localized injuries and 
trace them through their subsequent migrations, as Patterson 
(10) did in his gastrulation experiments, and thus obtain more 
conclusive evidence. 
COMPARATIVE DATA 
From the comparative point of view, there are several interest- 
ing aspects of these observations. Man is the only vertebrate 
species on record in which the otie primordium appears before 
the optic. The otic plate can be recognized at an extraordinarily 
early period—earlier, in fact than in any other form for which 
we have accurate data. Conversely, the optic anlage is differ- 
entiated relatively later than in most other mammals, if we 
take into consideration the fact that the earliest stage described 
in the literature corresponds to our fourth stage (fig. 9d). The 
optic sulcus can be identified in man at a slightly earlier stage 
than that in which the otic plate appears in other mammals. 
The following résumé includes only the available mammalian 
literature. 
Artiodactyls 
Bonnet’s (01) account of early sheep embryos gives only 
enough to make it clear that the optic primordium precedes the 
otic in this form. Neither was identified in his twelve-somite 
