A child is born without a fully developed pancreas. He is normal in all other respects, and thus doctors believe that the problem most likely began early in development. During which of the following processes did the original error most likely occur?

A) Mesoderm formation
B) Endoderm formation
C) Ectoderm formation
D) Cleavage

B. Endoderm Formation

Explanation:

Embryonic development is a 9 month long period. right after implantation, endoderm formation takes place.

The endoderm is one of the layers of germs that were developed during embryonic development and leads to development of the cells that line the organs.

During endoderm formation, tissue fusion takes place in which two opposing tissues come in contact and get fused to form one continuous structure. It  can affect the formation of many organs, including the palate, optic cup, heart and neural tube

Hence, the correct option is B.

phylogenetic tree is the tree of life or evolution tree. it shows the different branches of evolution. the sequence goes like tissues - coelom - backbone - endoskeleton - segmentation