The cycle of heating, rising, cooling and sinking is called a convection current. These currents are caused by the very hot material at the deepest part of the mantle rising, then cooling, sinking again and then heating, rising and repeating the cycle over and over.
<span>The answer is letter B. The formation of the three geminal layers and the primitive gut. The formation of gastrula starts when the cells starts folding in blastula slowly forming into a blastopore, that results to a double layer cup cell, forming the geminal layers and the primitive gut </span>
