While being propelled along the tube, segmentation or cell division takes place and the fertilized cell or zygote divides into 2-4-8-16 and so on until it consists of a ball of cells like a mulberry, known as the morula. Three to four days are required for the journey along the tube to the uterus, and as the part of the Fallopian tube through which it must pass has a diameter of 1 mm., the ovum must be very small to pass through it. A cavity or blastocele forms in the morula which now becomes known as the blastocyst.