108 



Mr. W. K. Parker. 



[Dec. 8. 



enclose a lanceolate space under the fore-brain (" thalamencephalon ") . 

 These somewhat flattened bands of cartilage become narrower up to 

 the middle of this large primary "pituitary space," and then recover 

 their former width in front, where they come in contact, having no 

 notochord between them. All the cartilage that lies in front of the 

 notochord is called trabecular ; between the trabecule, in front, there 

 is a small wedge of younger cartilage, the rudiment of the " inter- 

 trabecula." The hinder, or parachordal, parts are somewhat scooped 

 and bevelled above, and on their edge the auditory capsules rest. These 

 are quite distinct, and have a cartilaginous coat, which, however, has 

 a large oval deficiency below. As in the Batrachia, the fore part of 

 the "palato- quadrate " cartilages is continuous with the trabecules in 

 front; but the "pedicle" is free behind. The free " articulo- 

 Meckelian " rod is quite in front of the eye-balls, and is nearly as long- 

 as the hind suspensorium, or proper quadrate region ; this forward 

 position of the hinge of the mandible is not temporary as in the frog, 

 but is permanent. The uppermost element of the hyoid arch is an 

 anvil-shaped cartilage from the first, and ossifies afterwards, as the 

 hyo-mandibular and symplectic bones. As pointed out to me by 

 Mr. Balfour, its dorsal end is continuous, as cartilage, with the 

 skull (auditory capsule) above. The basi-hyal is not yet ossified, but 

 distinct inter-cerato- and hypo-hyal segments are already marked out. 

 Four larger and one small rods of cartilage are seen on each side, 

 articulating with a median band ; these are the branchial arches, which 

 chondrify before they undergo segmentation. In this stage there are 

 no osseous laminae as yet formed. 



Here, in this stage, in connexion with a large pre-nasal suctorial 

 disk, we have three important generalised characters, namely, the 

 continuity of the distal end of the mandibular pier and of the proximal 

 end of the hyoid pier with the skull, and the forward position of the 

 hinge of the jaw coupled with the horizontal direction of the suspen- 

 sorium. The hyoid arch has its segments formed much earlier than 

 in the Teleostei, and the " pharyngo-branchials " are not independent 

 cartilages, as in the Skate. 



The third stage — embryos two-thirds of an inch long — show a con- 

 siderable advance in the development of the skull ; the cartilage, 

 generally, is more solid and more extensive, and new tracts have 

 appeared. The apex of the notochord is now in the middle of the 

 basis cranii, for the prochordal tracts have grown faster than the 

 parachordals. The large so-called pituitary space is now irregularly 

 pyriform, and not lanceolate : the fore margin of the broad para- 

 chordal bands being now nearly transverse, whilst the prechordals 

 (trabecidce) are wide and fenestrate at first, then narrow, and then 

 widening suddenly, they coalesce, forming a sharp anterior end to the 

 pituitary space. 



