CHONDROCRANIUM OF SYNGNATHUS FUSCUS 431 



As in other non-siluroid teleosts, the trabecula communis is 

 considerably narrowed between the greatly enlarged eyes (figs. 

 2, 4). While in this region of a 5.7-mm. Gasterosteus the brain 

 lies immediately dorsal to the trabecula communis, in Syngna- 

 thus it is widely separated — a condition which may be ascribed 

 to the more rapid growth of the trabecula communis. 



The optic nerves cross through the interorbital septum in the 

 region of the optic chiasma a short distance dorsal to the trabec- 

 ula communis. The septum is distinct ventral to their crossing. 

 Posterior to this region the interorbital septum is broad and 

 indistinct, and it becomes less apparent as the ventral surface 

 of the brain approaches the trabecula communis in the region 

 just anterior to the fenestra myodomus ventralis. 



A cross-section of the trabecula communis in the interorbital 

 region has a distinctly double character, which indicates its 

 formation by the fusion of the anterior ends of the trabeculae 

 cranii. The anterior extent of the fenestra myodomus ventralis 

 is not as great in Syngnathus as in a 5.7-mm. Gasterosteus, 

 hence the trabecula communis is relatively shorter in the latter. 



A mass of diffuse mesenchyme cells, the primordium of the 

 vomer bone, is present along the ventral surface of the ethmoid 

 plate. More posteriorly a convex osseous lamella abuts against 

 the ventral surface of the trabecula communis. This is the 

 first ossification in the cranium and is the beginning of the 

 parasphenoid (fig. 4). It forms the median floor of the fenestra 

 myodomus ventralis and of the fenestra hypophyseos (figs. 5, 6). 

 No mention of this ossification is made in the description of the 

 early stages of the chondrocranium of Gasterosteus. 



Posterior to the orbital region, the trabeculae cranii diverge 

 to form the margins of the fenestra myodomus ventralis (fig. 5). 

 This fenestra is so named because of its homology to that space 

 in other teleosts, which lies between the trabeculae cranii and 

 anterior to the hypophysis, and lodges the proximal ends of the 

 recti eye muscles (AUis, '19). These eye muscles have their 

 origin on the dorsal surface of the parasphenoid lamella and are 

 not as yet separate elements. Posterior to the origin of the 

 recti muscles, the fenestral space is filled .with a dense mass of 



