THE DEVELOPMENT OF THE CHONDROCRANIUM IN THE SEAHORSE, HIPPOCAMPUS [LOPHOBRANCHII

Abstract

SUMMARY 1 Three stages in the development of the chondrocranium of Hippocampus are studied. In the last stage it is fully formed. 2 The ethmoid plate has an upward and backward bend in the 9mm. stage of Hippocampus. This flexure gradually straightens and the normal horizontal condition is assumed in the fully formed chrondrocranium. 3 The lamina orbitonasalis and the nasal septum exhibit their independent origin from the ethmoid plate by retaining their identity even after fusing with the ethmoid plate. 4 The orbital cartilages are reduced and are in the form of a pair of short processes projecting back from the laminae orbitonasales. 5 The trabecula communis becomes disconnected from the basal plate in the 12mm. stage. It is present in the fully formed chondrocranium as a small posterior process of the ethmoid plate. Hence the ethmoid region of the chondrocranium is independent of the occipito-auditory region. 6 A pila lateralis develops in the 18mm. stage enclosing the trigeminal nerve. In the earlier stages there is only a groove, the incisivum pro-oticum. 7 A lateral commissure is completely developed forming the ventrolateral wall to the trigemino-facialis chamber. 8 There is a minute basicapsular fenestra in stage 1. It becomes confluent with the metotic foramen in later stages. 9 The otic capsule does not show external prominences of the semicircular canals. There are no complete cartilaginous septa separating the semicircular canals. 10 There is no tectum synoticum. A slender tectum posterius connects the two occipital processes. 11 A vestigial taenia tecti medialis is the only roofing cartilage in the chondrocranium. 12 The pterygoid process of the quadrate is vertical in the 9mm. stage and it assumes a horizontal position in the 18mm. stage. The process is continuous with the body of the quadrate by means of a thin cartilaginous extension in early stages. The two parts become separated as the ethmoid plate is straightened. The pterygoid process which is at first fused with the ethmoid plate articulates later by means of a distinct rostropalatine process with the pre-ethmoid cornu conforming to the “ acrartete ” condition of Swinnerton's description. 13 The quadrate in the 9mm. stage is fused with Meckel's cartilage and separates from it later. 14 Meckel's cartilages are obliquely situated in the early stages and undergo a rotation of about 90 degrees to assume a horizontal condition along with the straightening of the ethmoid plate. 15 The hyosymplectic cartilage is elongated and extends to Meckel's cartilage and lies below the quadrate in early stages. In later stages the anterior end of the hyosymplectic lies behind the quadrate. The hyomandibula articulates with the otic capsule in front of the region of the lateral semicircular canal. A spheno-pterotic ridge overhangs the articular facet. 16 The copula communis in the fully formed stage does not extend beyond the first branchial arch. There are no basibranchial elements in the branchial arches. 17 The hypohyal and ceratohyal are distinct cartilages and undergo a change in position and size during development. 18 A well developed rostral cartilage is present on the dorsal surface of the distal end of the ethmoid plate from the early stages: It also moves with the ethmoid plate to assume an anterior position. 19 Infralabial cartilages are present in the form of reduced dense cellular masses. 20 Sclerotic cartilages are present in the optic capsules.