BY JAS. P. HILL AND C. J. MARTIN. 
55 
before the formation of the medullary folds. The same observer 
has also insisted on the fact that the neuromeric segmentation is 
primitively ectodermic and entirely independent of any meso- 
dermic segmentation — -a view with which the conditions in 
Platypus are in complete agreement. 
Auditory plates : The auditory plates are situated laterally to 
the neuromeric region of the hind-brain, their anterior edges being 
on a level with those of the second pair of neuromeres. Each 
consists of a thickening of the ectoderm which is distinctly 
grooved longitudinally (fig. 6, mid.). The appearance of the 
auditory plates in Platypus while the medullary plate is still flat 
is noteworthy. 
Cranial ganglia are not yet developed. However, in the 
interval between the third and fourth pairs of neuromeres and 
opposite the posterior portions of the auditory plates there occurs 
on each side a distinct downgrowth of the ectoderm just external 
to the outer edge of the medullary plate. This downgrowth is 
similar to the "Zwischenrinne" described by His* in 1879. Beard, f 
Rabl,| Chiurugi,§ and others have observed a similar structure, 
but do not regard it as concerned with the development of the 
cranial ganglia, G-oronowitsch,j| indeed, being of the opinion that 
it is an artificial production of the paraffin bath. 
There remains to be noticed a longitudinal strand of cells on 
each side lying close beneath the ectoderm just external to the 
edge of the medullary plate and passing slightly inwards below 
its outer margin (figs. 8 and 9, le.). The strands are found in the 
trunk region of the embryo extending from the first pair of somites 
anteriorly to about the fourteenth pair posteriorly. They are 
much larger and more distinct anteriorly. Whether these strands 
are ectodermal in their origin and ganglionic in their significance 
* His, Untersuchungen iiber die erste Anlage des Wirbelthierleibes — 
Leipzig. 
f Beard, Quart. Jo-urn. Micros. Sc. Vol. xxix. 
t Rabl, Morphol. Jahrb. Bd. xv. 
§ Chiurugi, Arch. Ital. de Biologie, Bd. xv. 
|| Goronowitsch, Morph. Jahrb. Bd. xx. Heft 2, p. 201. 
