Developmental stages of the chondrocranium in some selachians

Abstract

Serial sections of s e v~i i embryos were examined, i n four of which the cliondrocraiiia were reconstructed. l'aracliordals appcnr as independent precartilages on each side of the riotocliord i n the otie rrgiou. Later chondritieatiou occur^, nnd they fuse with the notochord. Orbitals and trabeculae appear as iiidepentlcut precartil⩾ later, as cartilages they fuse with the basal plate aiid with c a d i oilier, forininp; thr. cranial wall between the orbitul cavity uiid brain. S n alilr it shifting poster!orly of the bases of the fifth, seventh and eighth cranial nerves, in ect to the nntrrior end of the notochord a n d to t h i otic rraiclrs, occurs between the 10-inin. and the tiO-mni. xtnges. The otic vesicles, proportionately sinull i n tlie 10-nim. stage, reach their relatively largest &e in the 19-mm. embryo. The fio(irs of the otie capsules begin us lateral extensions of tlie basal plate a t the 25-mm. stage, and from these anlagen &~ndrifications extend, forming the anterior a n d posterior cuptilan.Most of the tcgmen cranii forms by cliondrifications t h a t approach the median plant! froin eucli side. I t is practically cwnplete i n the ti0-rnni. embryo. The cervical neural arches also contribute in forming the tegnicm cranii and the occipital region. The ventrally rrcnrved end of the notochnld nf the 10~1inii. stage persists tlirougli the 6O-mm. embryo. 565.JOURNAL OF MORPHOI.OOY AFD PHYSIOI.OGY, v m . 52, N o 2

Reconstructing the subcephalic musculature in Pucapampella and Ichthyostega

“…There is evidence of how the problem of elimination of unproductive application of pressure of the notochord onto anterior neurocranium unit across intracranial fissure was solved in ancestors of extant sharks. In their embryogenesis, the anterior end of notochord transiently forms a hook curved at 180° downward and backward (El‐Toubi, 1949; Harrison, 1931; Platt, 1891). To our knowledge, this unexpected curvature was never explained before.…”

Growth trajectories of prenatal embryos of the deep‐sea shark Chlamydoselachus anguineus (Chondrichthyes)

“…Earlier studies on the development of elasmobranchs have mainly focused on general descriptions of particular body parts and their anatomy (Balfour, 1881; El‐Toubi, 1952; Harrison, 1931; Holmgren, 1940; Jollie, 1971). However, only a few have attempted to present a staging system, which is useful for comparisons with other species (Ballard et al, 1993; Castro & Wourms, 1993; Didier et al, 1998; Rodda & Seymour, 2007; Maxwell et al, 2008; Onimaru et al, 2018).…”