The State Emblem was first adopted in 1823 and the eagle and snake have served ever since the Emblem of Arms of then successive republics and empires. It will be immediately apparent that the three hundred years of Spanish rule have been judiciously ignored, and in fact the Emblem recalls an old Indian legend: The Aztec people were guided by god Huitzilopochtli to seek a place where an eagle landed on a prickly-pear cactus, eating a snake... After hundreads of years of wandering they found the sign on a small swampy island in Lake Texcoco. Their new home they named Tenochtitlan ("Place of the Prickly Pear Cactus"). In A.D. 1325 they built a city on the site of the island in the lake; this is now the centre of Mexico City. The emblem was re-approved in 1934 and slightly modified in 1968; the plant is a nopal cactus.

The Coat of Arms was designed by Francisco Eppens Helguera, a famous Mexican Architect born in San Luis Potosi. The new coat of arms were granted by Decree of October 18, 1966, and officialy adopted on September 16, 1968.

Official description of the Mexican Coat of Arms

The coat of arms are described in the Article 2 of the "Ley sobre el Escudo, la Bandera y el Himno Nacionales" (Law on the National Coat of Arms, Flag, and Anthem) that reads:

The National Coat of Arms is featured by an Mexican eagle exposing its left profile, the upper part of the wings in a level higher than plume and slightly displayed in a battle attitude; with the sustenation plumage downwards touching to the tail whose feathers are arranged in natural fan. It puts its left grasp on a bloomed nopal that is born in a rock that emerges from a lake. It is grasping with the right grasp and the beack, in attitude of eat, a curved serpent, so that it harmonizes with the whole. Several "pencas" of the nopal grow to the sides. Two branches, one of encino to the front of the eagle and another one of laurel opposed, form a lower semicircle and they are united by a ribbon divided in three strips that, when the National Coat of Arms is represented in natural colors, correspond to those of the National flag.

When the National Shield reproduces in the reverse side of the National Flag, the Mexican Eagle will appear standing in its right grasp, holding with the left one and the beack the curved serpent.

See image bellow:

by Juan Manuel Gabino Villascán, 06 April 2001

by Juan Manuel Gabino Villascán, September 16, 2001

Source:
Decreto por el que se reforman los artículo 2o, 18 y 55, y se adicionan los artículos 54 Bis,... de la ley sobre el Escudo, la Bandera y el Himno Nacionales.
Issued in DOF on May 9, 1995.