Five types of constraints affect the stability of an  helix: 

(1) the intrinsic propensity of an amino acid residue to form an  helix; 

(2) the interactions between R groups, particularly those spaced three (or four) residues apart; 

(3) the bulkiness of adjacent R groups; 

(4) the occurrence of Pro and Gly residues; and 

(5) interactions between amino acid residues at the ends of the helical segment and the electric dipole inherent to the  helix.

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#Unit 5. Developmental Biology

In secondary neurulation, 

the neural tube arises from the aggregation of mesenchyme cells into a solid cord that subsequently forms cavities that coalesce to create a hollow tube.

 

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#Unit 5. Developmental Biology

Primary neurulation forms the anterior portion of the neural tube

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#Unit 5. Developmental Biology

Secondary neurulation forms the posterior portion of the neural tube 

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#Unit 5. Developmental Biology

In birds, primary neurulation generates the neural tube anterior to the hindlimbs.

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#Unit 5. Developmental Biology

In mammals, secondary neurulation begins at the level of the sacral vertebrae of the tail.

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#Unit 5. Developmental Biology

Junctional neurulation 

combination of mechanisms involved in both primary and secondary neurulation