THE CHONDROCRANIDM IN THE ICHTHYOPSIDA. 93 



Accompanying the general growth of the head and the 

 lessening of the cranial flexure, the trabeculte {t) have in- 

 creased considerably in size, more especially in length. 

 At their anterior ends they have become slightly broad- 

 ened and flattened to form the cornua trabeculae (c), the 

 first indications of the nasal capsules. The gaps between 

 the disconnected portions of the trabecular crest of the 

 first stage are now filled, forming a plate, the dorsal mar- 

 gin of which is a smooth, undulating line sloping gradu- 

 ually downward in passing from behind forwards. The 

 foramina for the optic and oculomotor nerves are now 

 completely enclosed. 



Several noteworthy changes have occurred in the quad- 

 rate (^). Its body and descending process have grown 

 broader and thicker, and two lateral projections from the 

 latter form a transitory support at the base of the bal- 

 ancer (sm). At the time of its most complete development 

 this support has the form of a shallow cup, the rim of 

 which coincides with the circumference of the base of 

 the balancer. The ascending process has extended up- 

 ward and has fused with the posterior end of the trabec- 

 ular crest. A slight backward projection from the body 

 of the quadrate is the first trace of the otic process. 



Third STAGE. — Larva twelve mm. long (Figs. 4-7). 

 The distal ends of the occipital processes {ocp) have 

 fused with the postero-dorsal walls of the otic capsules, 

 and their proximal ends have fused with the posterior ends 

 of the parachordals. A solid floor is thus formed beneath 

 the medulla, and the jugular foramen {j) is enclosed by 

 cartilaginous walls. 



The parachordals (p) with the notochord now form a 

 complete basilar plate below the posterior half of the 

 brain cavity except at one small place (/*) upon the left 

 side which still remains unchondrified. With the excep- 



