Answer: Blastocyst
Explanation: After fertilization the cell remains as it is with no cell division for the next 12 hours( single cell). Then after next 30 hours the single cell divides from 1 into 2 and then it divides into 4 after the next 15 hours.
At the end of 3 days the fertilized egg develops into a berry like structure which is known as morula made of of 16 cells.
Then these cell continues to divide further into blasocyst which is hollow pinhead structure. It consist of hundreds of cells.
