Illustrate the process of development during gastrulation in a chick embryo. (UPSC 2019, 15 Marks, )

चूजे के भ्रूण में गैस्ट्रुलेशन के दौरान विकास की प्रक्रिया को चित्रित करें। (Illustrate the process of development during gastrulation in a chick embryo.)