
To review:
The need for the two different mechanism for the formation of neural tube in relation to the evolutionary pressures that forced the adoption of secondary neurulation. The idea which influence the timing of end of gastrulation and its capacity for axis elongation in relation to neurulation.
Introduction:
The set of morphogenetic events that leads to the formation of neural tube is known as neurulation. This process acts as a base for the formation of central nervous system in different organism. Neurulation is characterized as a complex set of cell movements which includes epithelial columnarization, migration, intercalation and convergent extension. Multiple genes are involvesd in regulation of this process. Neurulation is accompained by two modes, primary neurulation and secondary neurulation.

Explanation of Solution
The formation of vertebral neural tube into anterior and posterior tube are two independent processes specific in morphogenetic and molecular processes. The anterior tube forms by primary neurulation, in contrast posterior tube forms via secondary neurulation. The main difference between these two processes is the role of epithelial and mesenchymal cell population. Epithelial tissues are an organized and continuous sheet of cells associated by junctional complexes whereas the mesenchymal tissue are loosely connected cells.
The evolutionary studies indicate that the mechanism of primary neurulation was the ancestral condition and the secondary neuraltion is evolved later like the evolution of limbs. Before the formation of neural tube, primary neurulation uses a flat neural plat whereas the secondary neurulation starts with a mesenchymal cell population which later condensed. The comparision of early stages serve to evaluate the evolutionary conservation. The precise movement in different vertebrates, shows a number of solutions to the problem of change in neural plate cells into a neural tube. So the morphological and antamoical changes with due course of time may be a reason for the evolution of secondary neurulation. The neural induction, at any given time during gastrulation is regulated by the different cells at the organizer and under the control of precise gene regulation. The cell division in the neural plate is preferentially in anterior-posterior direction, which also regulates the axis elongation during gastrulation.
Thus it is concluded that primary neurulation was the ancestral condition and the secondary neuraltion is evolved later like the evolution of limbs. The comparision of early stages serve to evaluate the evolutionary conservation. The precise movement in different vertebra may be a reason for this evolutionary condition.
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Chapter 13 Solutions
Developmental Biology
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