Question

Describe the slow evolutionary progression from the primitive notochord and nerve cord into a backbone and a spinal cord.

Answer 1

• Vertebrates are distinguished from other groups of animals by certain characteristics , principally related to their spinal cords and nervous systems, namely, the presence of a dorsal notochord surrounded by mesodermal tissue, which gives rise to a vertebral column in adults of most species, and a tubular nervous system lying dorsal to the cord, concentrated in the midline, and very well developed at its cephalic end (cephalization). The extent and pathways of nervous system development differs between invertebrates and vertebrates, even though, in all animals that have a nervous system, it is derived predominantly from ectoderm.
• Comparative embryology and anatomy have demonstrated various phases of the nervous system evolution. Specific aspects of evolutionary differences are :

A. PROGRESSIVE CONCENTRATION of the nervous system and begining of cephalization.

B. THE APPEREARANCE AND CHANGES IN SEGMENTATION seen in lower and superior annelids.

C. THE APPEARANCE OF NOTOCHORD AND NERVOUS SYSTEM in vertebrates.

• Invertebrate nervous system development: before the evolutionary appearance of the spinal cord, neural organization apparently depends on sensory surface ectoderm.

A. IN COELENTERATES : the cells are diffuse and the nervous system consists of a diffuse neural network.

B. IN PRIMITIVE WORMS : the sensory cells are concentrated in certain regions and the nervous system has a parallel development. It approaches a central nervous system organization, but it is ventral and without a cavity.

1. At its anterior end, sensory concentration is clear, and there is the beginning of cephalization with cerebral ganglia.

2. Nervous concentration is greatest anterior, and strict parallelism is seen between the anterior concentration of sensory and nervous cells.

C. IN LOWER AND LARVAL FORMS OF ANNELIDS : concentration has increased. Segmentationor metamerisation appears. The central nervous system is ventral, has no cavity and localization of sensory cells seems to be responsible for the segmental concentration which forms the metameric ganglia.

D. THE SUPERIOR AND ADULT ANNELIDS: the central nervous system is still ventral (single cord and ganglia) without a cavity, and there is transverse concentration. The cerebral ganglia are specialized for vision and oflaction.

E. THE HEMICHORDATES (enteropneusts): a dorsal tubular and a solid ventral nervous system. There is no cord, but there is a cephalodorsal diverticulum of the intestine corresponding toa dorsal nervous tube. The diverticulum is incompletely seperated from the overlying ectoderm from which it is derived.

F. THE PROCHORDATES : a dorsal cord and a dorsal tubular central nervous system entirely developed  from dorsal ectoderm. A ventral system is no longer seen. The anterior vesicle represents a true rudimentary brain.

• Vertebrate nervous system (Chordata): mesoderm determines the development of the nervous system and sense organs. The appearance of mesoderm groups together the inductive capabilities and marks an important stage in the genesis of development.

A. THE NERVOUS SYSTEM is relatively independent of the sensory organization. However, the eye and general sensory receptors, are directly derived from the nervous system.

1. During evolution, relationships between the nervous system and the sense organs are modifie In invertebrates, the nervous system appears to be only an accessory structure to the sensory system. In vertebrates, the importance of the nervous system becomes so great that everything else is organized for it and around it.

1. B. IN VERTEBRATES SUCH AS FISH, the 5 neural vesicles presage the cephalization of the higher vertebrates. During the slow progression of evolution, the role of the prochordal plate becomes essential since, in humans, it induces brain formation.
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