Summary.1 Eight stages in the development of skull elements, commencing from the first day to twenty‐five days after hatching, have been studied.2 The mesenchymatous plate in contact with the ectoderm on either side of hypophysis is seen in stage 1 (2.75 mm.).3 The trabeculae and parachordals arise independently in stage 2 (4.0 mm.).4 The trabeculae are joined to the parachordals, leaving a wide hypophysial fenestra in stage 3 (4.0 mm.). The visceral arches chondrify and the first two branchial arches appear in this stage. The basibranchials have fused to form a copula communis.5 In stage 4 (4.5 mm.) the rudiments of the occipital arches are noticed, A basicapsular fenestra is seen. The pterygoid process is noticed as an independent chondrification as inClupea, and the quadrate is fused with the hyomandibula, as also inClupea. All the five branchial arches have developed inOphicephalus. The fifth branchial arch extends to a few sections.6 In stage 5 (5.0 mm.) the pharyngobranchials have developed in connection with the first three branchial arches. The metotic foramen is delimited by the fusion of the occipital arch with the auditory capsule posteriorly. Meckel's cartilages have fused anteriorly at the symphysis.7 In stage 6 (5.0 mm.) the pterygoid process is connected with the ethmoid plate by connective tissue.8 In stage 7 (6.0 mm.) the pterygoid process extends posteriorly and is fused with the quadrate. An interhyal fused with the hyomandibula is noticed to be in contact with the ceratohyal.9 In the fully formed chondrocranium, stage 8 (24.0 mm.), the rostral cartilage is developed and is not fused with any part of the ethmoid plate. The lamina orbitonasalis develops on each side of the middle region of the ethmoid plate. There is a small process directed posteriorly at the point where the lamina orbitonasalis has fused with the nasal septum, representing the anterior portion of orbital cartilage. The ophthalmic branch of the trigeminal nerve passes in a groove dorsal to the lamina orbitonasalis. The trabeeulae disorganize and are represented as a small extension posterior to the trabecula communis. The hypophysial fenestra is obliterated. The two orbital cartilages are united in the middle region of the chondrocranium by an epiphysial bar. A small taenia tecti medialis extends posteriorly from the middle region of the epiphysial bar. The trigemino‐facialis chamber has no complete lateral commissure. The basicapsular fenestra and the metotic foramen are separated by the basivestibular commissure. The two auditory capsules are joined by a tectum synoticum. There is no tectum posterius. There are three prominences externally on the auditory capsules for the accommodation of the three semicircular canals. The hyomandibular branch of facial nerve passes in a foramen in the hyomandibula. The quadrate is fused with the hyomandibula. The hyomandibula is connected with the ceratohyal by a stylohyal. The pterygoid process is discontinuous with the quadrate. Meckel's cartilage articulates with the quadrate and there is a retroarticular process of Meckel's cartilage.In the branchial arches the basihyal is continuous with the copula communis. The ceratohyal shows a stout hypohyal in close contact with the basihyal. There is a foramen in the hypohyal for the hyoidean artery, recorded only inSolea. The first four branchial arches show differentiation into the hypo‐, the cerato‐, the epi‐and the pharyngobranchials. The fifth branchial arch is small and consists of a single element—the ceratobranchial. The pharyngobranchials of the third and fourth arches are fused.10 There are two small rods in connection with the second branchial arch ventrally on either side of the copula communis. I have called these the sub‐copular rods. The homology of these rods is not clear.11 In the chondrocranium the ethmoid region is completely separated from the auditory region by the discontinuity of trabeculae, the orbital cartilages and the pterygoid processes.12 The chondrocranium is platytrabic,