From Style in Guide part of ABC of Plain Words by Sir E Gowers (1951)

The purpose of a parenthesis is ordinarily to insert an illustration, explanation, definition, or additional piece of information of any sort, into a sentence that is logically and grammatically complete without it. A parenthesis may be marked off by commas, dashes or brackets. The degree of interruption of the main sentence may vary from the almost imperceptible one of explanatory words in apposition:

Mr. Smith, the secretary, read the minutes,

to the violent one of a separate sentence complete in itself:

A memorandum (six copies of this memorandum are enclosed for the information of the Board) has been issued to management committees.

Parentheses should be used sparingly. Their very convenience is a reason for fighting shy of them. They enable the writer to dodge the trouble of arranging his thought properly; but he does so at the expense of the reader, especially if the thought that he has spatchcocked into the sentence is an abrupt break in it, or a long one, or both. The second of the two examples just given shows an illegitimate use of the parenthesis. The writer had no business to keep the reader waiting for the verb by throwing in a parenthesis that would have been more appropriate in a separate sentence. The following examples are even worse:

. . . to regard day nurseries and daily guardians as supplements to meet the special needs (where these exist and cannot be met within the hours, age, range and organisation of nursery schools and nursery classes) of children whose mothers are constrained by individual circumstances to go out to work. . . .
If duties are however declined in this way, it will be necessary for the Board to consider whether it should agree to a modified contract in the particular case, or whether — because the required service can be provided only by the acceptance of the rejected obligations (e.g. by a whole-time radiologist to perform radiological examinations of paying patients in Section 5 beds in a hospital where the radiologists are all whole-time officers) — the Board should seek the services of another practitioner. . . .

These are intolerable abuses of the parenthesis, the first with its interposition of 21 words in the middle of the phrase "needs of children", and the second with its double parenthesis, more than 40 words long, like two snakes eating each other. There was no need for either of these monstrosities. In both examples the main sentence should be allowed to finish without interruption, and what is now in the parenthesis, so far as it is worth saying, should be added at the end:

. . . to regard day nurseries and daily guardians as supplements to meet the special needs of children whose mothers are constrained . . . and whose needs cannot be met. . . .
. or whether the Board should seek the services of another practitioner, as they will have to do if the required service can be provided only . . .

Here is a parenthesis that keeps the reader waiting so long for the verb that he has probably forgotten what the subject is:

Close affiliation with University research in haematology — and it may be desirable that ultimately each Regional Transfusion Officer should have an honorary appointment in the department of pathology in the medical school — will help to attract into the service medical men of good professional standing.

In former days, when long and involved periods were fashionable, it was customary after a long parenthesis to put the reader on the road again by repeating the subject with the words "I say". Thus the writer of the last example would have continued after "medical school" with the words

"close affiliation with University research in haematology, I say, will help to attract, etc."

Now that this handy contrivance has fallen into disuse, there is all the more need not to keep the reader waiting. There was no necessity to do so here. What is said as a parenthesis might just as well have been said as an independent sentence following the main one.

It is not only the reader who may forget where he was when the parenthesis started. Sometimes even the writer does.

..Owing to a shortage of a spare pair of wires to the underground cable (a pair of wires leading from the point near your house back to the local exchange, and thus a pair of wires essential to the provision of a telephone service for you) is lacking. . .

The writer thought he had entered the parenthesis with the words "Owing to the fact that a spare pair of wires to the underground cable" and he continued conformably when he emerged.