CHONDROCRANIUM OF A 20 MM. HUMAN EMBRYO 621 



14 mm. embryo of Levi they appear larger proportionately than 

 in this 20 mm. model, and they show successive decrease through 

 his series. We may also conclude that the dorsal extensions of 

 the parietal plates which result in the formation of the tectum 

 synoticum share in the temporary nature of the rest of the 

 structure. 



In my description of the otic capsule, I have placed the model 

 with the vertebral column vertical and the basal plate hori- 

 zontal. After the rotation of the basal plate takes place, the 

 capsule being carried with it, the ventral pole becomes cranial, 

 the cranial pole dorsal. This occurs between the 20 mm. and 

 28 mm. stages. The change accounts for my using different 

 terms of position from those of Macklin for the same points. 



The orbito-temporal region consists of the sphenoid body 

 mesially, and two processes on each side, the alae temporales and 

 the alae orbitales (fig. 1). 



I have already described the manner in which the planum 

 basale thickens as it approaches this region. From the crista 

 transversa the prominent dorsum sellae projects cranially and 

 dorsally (fig. 2, 7). This appears to be firmly united to the 

 crista transversa by continuous chondrification, not separated 

 as described by Fawcett ('10), in the 19 and 21 mm. stages. 

 Cranial to this ridge the surface is hollowed by a transverse gut- 

 ter, the sella turcica (figs. 1 and 2, 8). The floor of the sella 

 turcica is formed by the expanded cranial end of the basal plate. 

 Cranial to the sella, the surface rises dorso-craniad to form a 

 limiting wall of the sella in this direction (fig. 1). This wall 

 should be noted, as Levi showed that in the 17 mm. stage, the 

 sella had no wall cranially, making one surface with the lamina 

 hypochiasmatica. The lamina hypochiasmatica (figs. 1 and 2) 

 is a broad flat surface in front of the sella turcica, on a different 

 level. This change of level Levi found established in his 28 mm, 

 embryt*', the next oldest in his series. It is however perfectly 

 developed in this embryo of 20 mm. Fawcett does not describe 

 it. Levi makes the point that the floor of the sella retains its 

 original position w^hen the planum basale heaves up and the 

 'amina hypochiasmatica is elevated. It is interesting to note 



JOURNAL OF MORPHOLOGY, VOL. 27, NO. 3 



