ectoderm

"The outermost layer of the onion was paper-thin. "

"The development of this story has been slow."

"The dry-stone wall used a large, flat through to support the top stones. "

"The scientist studied the embryonic development of the frog. "

"The sunburn only affected the epidermis, so it wasn't too deep. "

"The taste of salt differentiates the soup from plain water. "

"The delicate tissue paper wrapped the fragile antique doll. "

"The dancer's nervous muscles tensed before her performance. "

"The scientist studied the embryo to understand how the early stages of development occur. "

"After the first coat of paint dried, he applied another layer."

"The farmer's fields produced a bountiful harvest of tomatoes this year. "

"The biology textbook explained that a human, being an animal developing from an embryo with three distinct tissue layers, is classified as a metazoan. "