题目

    Elizabeth Blackwell was born in England in 1821, and moved to New York City when she was ten years old. One day she decided that she wanted to become a doctor. That was nearly impossible for a woman in the middle of the nineteenth century. After writing many letters asking for admission(录取) to medical schools, she was finally accepted by a doctor in Philadelphia. She was so determined that she taught school and gave music lessons to get money for the cost of schooling.     In 1849, after graduation from medical school. she decided to further her education in Paris. She wanted to 下列各项中，哪项不是皮肤生发层所具有的特点（　　）A．细胞经常脱落B．没有血管C．有黑色素细胞D．细胞具有分裂能力