330 CHARLES CLIFFORD MACKLIN 



stages and by Froriep in the 17.5 mm. stage, the latter having 

 found the planum broken by a gap, filled with perichondrium, 

 1.9 mm. behind the canal of the hypophysis. This was just 

 dorsal to the region where the occipitopharyngeal ligament, 

 which had disappeared in older embryos (Levi), was inserted. 

 This primitive gap (of which there is no evidence in my model, 

 except the relative thinness of the cartilage in this region — ■ 

 and in this my findings agree with those of Levi and Jacoby 

 for the 28 mm. and 30 mm. stages respectively — marks the site 

 of the division which exists in early stages, between the two 

 constituent parts of the planum, viz., that lying below it, be- 

 longing to the occipital region and representing the anlage of 

 the future basi-occipitalis, and that lying above it, the clivus 

 of Blumenbach, which is destined to form the basi-sphenoidalis, 

 and has been included in the otic region. 



The connection of the clivus with the body of the cartilaginous 

 sphenoidal anlage is very primitive, according to the investi- 

 gations of Levi upon young human embryos, as in the 13 mm. 

 stage it is present while as yet there is no cartilaginous otic cap- 

 sule in existence. This circumstance, combined with the re- 

 lationship of the clivus to the basi-sphenoidalis of the osseous 

 condition would seem to indicate that the upper portion of the 

 planum should be grouped with the regio orbitotemporalis 

 rather than with the regio otica, and this has been done by Levi, 

 Jacoby and Van Noorden, but since Gaupp has more recently 

 shown that the upper portion of the planum is more properly 

 included with the otic region, and as several authors have since 

 followed this course for the mammals (Voit, Mead) I shall adopt 

 it in this description. The more minute details of the occipital 

 and otic portions of the planum will be considered in the dis- 

 cussion of these regions. 



The material composing the planum basale is mature cartilage, 

 of uniform character, excepting one small, isolated mass of' en- 

 larged, vacuolated, cartilage cells, the nuclei of which appear 

 larger and more darkly-staining than those of the surrounding 

 areas, and the ground-substance of which has stained a dark 



