Moving to cement its planned takeover of Euronext NV, the New York Stock Exchange’s parent made an important concession in the structure of the combined company by agreeing to an even split in the number of directors from each side of the Atlantic.

NYSE Group Inc. shares were recently up 8.7% at $103.60. The stock crossed the $100 level for the first time Tuesday, setting an all-time high at $104.66.

In a statement, the NYSE and Euronext said the 22-member board of NYSE Euronext, as the joint company will be called, will be staffed by an equal number of American and European directors. The merger would join the 214-year-old Big Board with a large pan-European stocks and derivatives exchange.

Previous plans had called for the NYSE to have an edge in the number of board seats, which didn’t sit well with some European critics of the deal’s structure. Gerard Mestrallet, a French executive who is the chairman of Paris Europlace, a French lobbying group, said last month that he favored an even split on the NYSE-Euronext board. Neither Mr. Mestrallet nor a Paris Europlace official in France could immediately be reached for comment Tuesday.

While the NYSE-Euronext merger still has hurdles to clear — namely, winning regulatory approval and securing the backing of Euronext’s shareholders — the board concession could help NYSE Chief Executive John Thain assuage critics and close the deal.

Thain has already scored one important victory by outlasting another suitor that had been interested in Euronext and that recently gave up, Frankfurt’s Deutsche Boerse AG. Shareholder meetings on the transaction are expected for December.