A peculiar feature of the embryonic mammalian circulatory system is that in the area of the heart the cells adhere to one another, beating in unison and adopting specialized orientations exclusive of one another.


(A) beating in unison and adopting

(B) they beat in unison while adopting

(C) beat in unison, and adopt

(D) beating in unison yet adopting

(E) even though they beat in unison and adopt

"peculiar" signals that the sentence should carry the original meaning and then deviate at a point.

D it is...."yet" is the word which does this and also is parallel

This is a tricky one. A cursory read may lead one to infer this as a parallelism error in which case the correct answer would be C.

The singular 'feature' suggests that only one feature is being described, namely the cells adhering to one another. The second part of the sentence elaborates on what the 'pecularity' is. The pecularity is that even though the cells beat in unison they adopt orientations exclusove of one another. D correctly brings out this contrast by using the word 'yet'.

The correct answer is D.

Here's my take on this question w/out repeating too much of what other people's said.

Since the last part of the sentence said, "beating in unison and adopting specialized orientations exclusive of one another.", we should see a contrasting triggering word to differentiate between "unison" and "exclusive of one another". Answer choices A, C, and E all used "and", for which it doesn't trigger the contrasting effect. Only B and D uses the right words - while (B) and yet (D).

Looking into B and D, B creates a common splice. In (B), "they beat in unison..." creates a complete sentence with a subject and a verb. In conjunction with the non-underlined sentence, the comma in between the two creates the grammatical error; therefore, (B) is incorrect. (D) fixes all the problems of this sentence.

In response to MrEasy's question with the comma after the word 'heart', I don't believe the comma is necessary since the material after the word 'heart' is a necessary clause, which requires no comma in between the two clauses. I hope I didn't confuse anyone with what I said. Also, just to go aside from that, people get bogged down by parts that are not underlined way too much; everyone should take what's given to them (the non-underlined portion) knowing that it's correct and don't question the non-underlined portion. I just wanted to point that side point out, though I'm pretty sure most people on this forum already knew that.
_________________

Since people have doubts about B and C :

Notice that there is no conjunction connecting the two parts of the sentence; so the second part can't be a clause!!! It can only function as participle modifiers.

B: Usage of they converts it to a clause. So this is out
C: beat is a verb, not a modifier. Only verbs in participle form can act as modifiers

For these reasons, B and C are out

The only confusing options are A and D.

Here the sentence is trying to bring out the contrasts... so we need yet. Hence A is out. D is the answer
_________________

Just look at the original text especially the non-underlined part. It says ' and adopting'. Therefore we need another structurally parallel modifier before the 'and'. One can safely remove B, C, and E for using a verb' beat'.
It is now easy to converge on D. we want a contrast and that is shown by the word yet. A is gone.
_________________