. Chordate morphology. Morphology (Animals); Chordata. eye muscles enter anterior myodome /1^ olfactory nerve (I) lateral ethmoid nerves ond blood vessels interorbital septum optic pedicel Figure 5-6. Snout part of the adult endocranium showing the anterior myodome; the roof of the right half has been cut away (cross hatching indicates surfaces). canal. In still other bones, the basal plate appears at much the same time as the canal organ center. In contradiction to the concept of sensory organ of tooth origin of bones is the fact that not all bones of the fish arise in relation to one of thes
. Chordate morphology. Morphology (Animals); Chordata. eye muscles enter anterior myodome /1^ olfactory nerve (I) lateral ethmoid nerves ond blood vessels interorbital septum optic pedicel Figure 5-6. Snout part of the adult endocranium showing the anterior myodome; the roof of the right half has been cut away (cross hatching indicates surfaces). canal. In still other bones, the basal plate appears at much the same time as the canal organ center. In contradiction to the concept of sensory organ of tooth origin of bones is the fact that not all bones of the fish arise in relation to one of these sources; further, there is experi- mental evidence that bones will develop, somewhat later perhaps, after the surgical removal of the sensory organs. Also, in many animals sensory organs are never developed but bones arise much as the basal plate of the fish canal bone. It has long been assumed that the sensory organ, as a more differentiated structure lying in a canal and enclosed by a concentration of connective tissue, presents an area with a higher metabolic rate and substrate conditions that might serve as a first condensation point. There is little question that in fishes the sensory organ centers are char- acteristic and useful in determining homologies. The origin of the maxilla and dentosplenial is of sjjecial interest. A strand of mesenchyme extends out and back from the anterior end of the palatoquadrate (Figure 5-9). This strand posteriorly is associated with Meckel's cartilage and its end grows down and forward with that structure. The strand now lies in both upper and lower lip margins and appears to give rise to the maxilla above and the dentosplenial below. Anterior to the tip of the maxillary blastema, a premaxil- lary blastema condenses from the mesenchyme underlying the snout. Behind this, pairs of vomer and palatine anlages appear. Behind the dentary, an angular blastema forms. All of these blastemas are similar and represent bone origins that cannot be di
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