According to scientists, recent studies says about the pattern of enamel prism demineralization, true is:
Head and tail both are resistant to dissolution
Head and tail both are not resistant to dissolution
Head is more resistant to dissolution
Tail is more resistant to dissolution
Enamel is mainly composed of millions of enamel rods or prisms, sheaths and a cementing inter rod substance.
Each rod has a head and tail. 
The head is directed occlusally and the tail is directed cervically. 
Basically, acid etching creates a 5 to 50 micron deep microporous layer into which adhesive resin flows. 
This results in a long-lasting enamel bond achieved via micromechanical interlocking between the resin and enamel.
Etching produces irregular and pitted surface with numerous microscopic undercuts with uneven dissolution of heads and tails, the latter being more resistant.

Ref: Textbook of Operative Dentistry, Nisha and Amit Garg, 3rd edition, Pg no:28,237