A scientist is studying some developing eggs from an ocean sample. The early embryos have divided only a few times, and all cells look identical. When the scientist carefully removes half the cells from an embryo, both clumps of cells are able to complete development, forming twins. The eggs can be identified as:

Question

A.  deuterostomes
B.  protostomes
C.  crustaceans
D.  cnidarians
E.  echinoderms

Clarify Question
· What is the key concept addressed by the question?
· What type of thinking is required?

Gather Content
· What do you already know about determinate and indeterminate development?

Choose Answer
· Given what you now know, what information and/or problem solving approach is most likely to produce the correct answer?

Reflect on Process
· Did your problem-solving process lead you to the correct answer? If not, where did the process break down or lead you astray? How can you revise your approach to produce a more desirable result?

Answer 1

A.  deuterostomes

Clarify Question
· What is the key concept addressed by the question?
        o This question addresses determinate and indeterminate development.
· What type of thinking is required?
        o This question is asking you to weigh and judge evidence, or evaluate, to choose the best of the possible answers.

Gather Content
· What do you already know about determinate and indeterminate development?
        o Many protostomes exhibit determinate development, in which the type of tissue each embryonic cell will form in the adult is determined early, in many lineages even before cleavage begins, when the molecules that act as developmental signals are localized in different regions of the egg. Consequently, each embryonic cell is destined to occur only in particular parts of the adult body, so if the cells are separated, development cannot proceed.
        o Deuterostomes, conversely, display indeterminate development. The first few cell divisions of the zygote produce identical daughter cells. If the cells are separated, any one can develop into a complete organism (this is how identical twins are formed). Thus, each cell remains totipotent and its fate is not determined for several cleavages.

Choose Answer
· Given what you now know, what information and/or problem solving approach is most likely to produce the correct answer?
        o Since the embryos could be split in half and still form normally, this indicates they have indeterminate development.
        o Indeterminate development is a feature of deuterostomes. However, based on just this information we can not identify what type of deuterostome.

Reflect on Process
· Did your problem-solving process lead you to the correct answer? If not, where did the process break down or lead you astray? How can you revise your approach to produce a more desirable result?
        o The question required you to weigh and judge evidence, or evaluate, to choose the best of the possible answers.
        o Did you recognize that the split embryos showed indeterminate development, a characteristic of deuterostomes?