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 auditor}^ 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, aud.). 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, Goronowitsch,|| 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. 



i Beard, Quart. Journ. Micros. Sc. Vol. xxix. 



:|: Rabl, Morphol. Jahrb. Bd. xv. 

 § Chiurugi, Arch. Ital. de Biologie, Bd. xv. 

 II Goronowitsch, Morph. Jahrb. Bd. xx. Heft 2, p. 201. 



