518 H. H. SWINNERTON. . 



epithelium shows no signs of active gi-owth even in younger 

 individuals of this stage. In the articular region, however, 

 this is rich in nuclei, and appears to be actively giving rise 

 to the substance of the thick fibrous sheath externally and to 

 cells internally; an appearance which suggests that cells from 

 this region may have crowded forwards. Such an interpre- 

 tation is suppoi'ted by the fact that those cell walls which are 

 attached to tlie chorda! sheath all have their inner portions 

 inclined forwards, as though there had been a movement in the 

 same direction on the part of the central cells. Externally it is 

 completely enclosed in a casing of a bone — the basioccipital, — 

 whose relation to the notochord was accurately described by 

 Huxley, and compared by him with the ui'ostyle (58, p. 440). 



Apart from the appearance of numerous ossifications, the 

 changes which at this stage have taken place in the chondro- 

 cranium are few (PI. 28, fig. 9). Owing to the suppression of 

 part of the supra-orbital bands, and of those parts of the 

 trabeculee which border on the pituitary fossa, the two parts 

 of the chondrocranium which are essentially tiabecular and 

 parachordal in origin have become completely separated from 

 one another. 



In the hinder or parachordal portion the interparachordal 

 fossa (fig. 12, 2). ch.') has been carried so far away in front of 

 the notochord that the plate formed by the median union 

 of the parachordal now furnishes a considerable portion of 

 the basis cranii. Those parts lyiug immediately on either 

 side of the fossa have now begun to undergo a movement 

 of depression, by which they have already come to lie 

 slightly below the level of the basis cranii. 



This is perhaps associated with a similar movement on the 

 part of the recti muscles. In the previous stage the inner ends 

 of these were inserted into one another and into the tissue which 

 fills the hinder part of the fossa. Owing to the point of their 

 insertion on the eyeball (fig. 58, m. e.) being in a plane situated 

 a little in front of this, but considerably above that of the basis 

 cranii, their course outwards over the anterior ends of the para- 

 chordals was obliquely upwards. In this stage, however, their 



