520 H. H. SWINNERTON. 



the stickleback is now identical with that described and 

 figured by Walther (82, p. 10) for the young pike, and 

 Pouchet (78) for Engraulis. 



All the ossifications present in the adult are now repre- 

 sented, but as yet are widely separated by cartilaginous areas. 



The basioccipital (fig. 12, oc. h.), owing to the hinder 

 funnel-shaped enlargement of the notochord, has begun to 

 take on the usual centrum-like condition. At its extreme 

 anterior edge the ventral lamina of bone leaves the cartilage 

 and projects freely forwards, as indicated by the yellow 

 pointed portion in fig. 12, and is better understood from the 

 longitudinal section of this region of the adult (PI. 30, fig. 37, 

 ey. c). This free portion lies between the posterior pro- 

 longations of the parasphenoid (fig. 36, j). s.), and is formed 

 by ossification in connective tissue. 



The vagus and pneumogastric nerves pass out through the 

 exoccipital (figs. 9, 12, oc. e.) by a common foramen, as in the 

 previous stages ; but in the individual figured the pneumo- 

 gastric nerve of one side passed out by a small foramen, 

 separated from that of the vagus by a narrow bridge of bone. 

 This is the beginning of a process which continues on both 

 sides, and ultimately carries the two foramina far from one 

 another. 



Posteriorly the exoccipital, by extending into the sur- 

 rounding tissue, has enclosed the first spino-occipital nerve 

 (fig. 9, oc. n.') completely, and the second [oc. n.") partially. 



The supra-occipital (fig. 4, oc. s.) extends along the strip 

 of cartilage which joins the posterior cranial roof to the 

 epiphysial cartilage, almost to the level of the post-orbital 

 processes. The ossification does not yet affect any part of 

 the epiphysial cartilage itself, but it extends laterally into 

 regions where no cartilage exists or has existed during 

 ontogeny ; that is to say, into the connective tissue which 

 roofs over the fontanelles. It cannot therefore be regarded 

 as a simple cartilage bone; but it may be that this lateral 

 extension is indicative of a former greater extension of the 

 cartilage. 



