The process of gastrulation forms the
blastula
zygote
organs
germ layers

Germ layers

Explanation:

Gastrulation is the process of formation of the three germ layers from a blastula. The gastrulation starts after the 3rd week of fertilization. In this process, the blastula forms a bilayer disc. This bilayer disc consisting of outer epiblast and inner hypoblast. Then this bilayer disc transforms into three layered discs.  

These three layers are ectoderm, mesoderm, endoderm, and are the 3 germ layers. The ectoderm develops from the epiblast, endoderm arises from the hypoblast, and the mesoderm is the middle layer which develops in between the epiblast and hypoblast. The ectoderm is the outer layer and the endodermis the inner germ layer. Then different organs develop from these three germ layers.

i believe it is "they are not used up by reactions" as the answer to your question, text me back if this is correct because i just learned this.

easy

explanation: