OTIC AND OPTIC PRIMORDIA IN MAN 227 
ganglia, but the otic plate is not mentioned. It is possible that 
the latter appears early in the cat, although it certainly does not 
precede the optic anlage as in man. 
The neural crest deserves further study in the carnivores. 
Weigner found no evidence for mesectoderm formation in the 
ferret and Schulte and Tilney affirm that all cells which leave 
the neural folds enter into the cranial ganglia. Martin (p. 342) 
found ‘neural crest’ beginning ‘dicht hinter’ the optic vesicle 
and extending through the midbrain giving rise to the sensory 
components of the third, fourth, and part of the fifth cranial 
nerves. If the microscopic picture in these forms is really not 
complicated by a mesectodermal proliferation, they are partic- 
ularly favorable material for the study of the origin of the 
cranial ganglia and especially of such problems as the origin and 
fate of the muscle sense cells of the oculomotor nerves. On the 
other hand, it is also possible that the particular embryonic 
stages during which the mesectoderm migrates out have not yet 
been studied. It would seem that in the cat the nervous system 
is differentiating more rapidly than the axial mesoderm and that 
a close series, so far as the number of somites is concerned, may 
have distinct. gaps in it. In man the period of pro-otic mesecto- 
derm formation is limited to stages between eight and twelve 
somites, and it may therefore be that this phenomenon occurs 
between five- and seven-somite stages in the cat—a period which 
was not represented in the Columbia University series. 
For the dog we have the descriptions of Bischoff (45) and 
Bonnet (’01). Figure 36 of the former shows an embryo of about 
ten somites with obvious optic vesicles. The latter’s figure 
39 is taken from a section of an eight-to-nine-somite specimen 
and seems to pass through the edge of the optic primordium. 
Bonnet’s ten-somite embryo has both optic vesicles and otic 
pits. Here, then, as in the cat the two primordia arise at about 
the same time, the optic, however, taking precedence. 
Rodents 
When one considers the large collections of rodent embryos in 
many embryological laboratories, it is surprising that so little 
