200 ME. W. K. PAEKEE ON THE STEUCTUEE AND DEVELOPMENT 



The most advanced specimens at this stage (1 inch 4 lines long) show much that is 

 instructive, and fairly bridge over the space between the first and third stages. 



The head is fast becoming straight — see the nearness of the fore margin of the nasal 

 sacs to the front of the head (PL XXXV. figs. 6, 7, as compared with fig. 5) ; and the 

 trabeculse, investing plates, notochordal sheath, auditory capsules, and visceral arches 

 are all now chondrified. 



In a horizontal view of a preparation iu which the nasal and optic capsules were cut 

 through, and the brain all removed, except in front (PI. XXXV. fig. 6), we get a good 

 view of the foundations of the Selachian chondrocranium. 



The notochord, which with the investing mass that has been cut through at some dis- 

 tance behind the auditory capsules, is enclosed in a strong sheath of hyaline cartilage, 

 has lost its beaded character in front, and now has pressed its end fiat against the back 

 of the pituitary body. 



The halves of the investing mass are scooped along their uiner edges, where they 

 cling to the sides of the notochord. Each plate passes some distance below the auditory 

 capsule, but much more at both ends than in the middle. These cartilaginous bands 

 have coalesced with the trabeculee in front, growing into the lower edge of the thick 

 transverse postpituitary wall {p.cl, py, iv, tr). 



The auditory capsule inside the anterior ampullar enlargement has coalesced with the 

 thick outer end of the posterior clinoid wall. In front of the wall the trabeculae dip, 

 and are somewhat concave ; behind, directly in front of the ear-capsules, each trabecula 

 is growing upwards into an alisphenoidal crest, which runs forwards to the optic 

 nerve (2). This is their narrow part ; further forward they expand behind the nasal sacs 

 in a pedate manner, but do not yet meet at the mid line. 



The intertrabecular space is larger than its enclosing cartilages, and is only occupied 

 at its end by the neck of the pituitary body. 



At the mid line, between the trabeculse and the olfactory sacs, the granular semi- 

 cartilaginous internasal tract is seen. I cannot discover that the tract is ever divided 

 into two distinct bands of cartilage, although its counterpart always is so divided, below, 

 in the Amphibia ; in front it widens, curves right and left round each nasal sac for some 

 distance, and in the middle sends forward an azygous rod. 



This latter is the praenasal rostrum, the axis of the " cutwater;" and the lateral 

 growths are the cornua trabeculse. Each of these is bilobate ; and in the next stage we 

 shall see what a curious modification these two projecting masses of cells have 

 undergone. 



At present it should be noted that the olfactory sacs, whose dome-like roof is now 

 fast changing into cartilage, are very close together, only leaving a narrow valley 

 between them, and leaving scant room for the septum or internasal region of the 



trabeculae. 



When the head is examined from below we see the free forward growth of the 



