I was a bit more general in saying "Developmental Biology" and integrated my personal experience into that. Realistically, as long as I'm doing Developmental Bio research, I'm happy. Only schools where they wanted or encouraged me to name labs did I mention any prospective thesis advisors.
As long as you're able to summarize your past experiences and transition into the present, writing a page or 500 words shouldn't be too difficult. I had to cut out a lot of "wordy" parts as I am a rather flowery writer and make it more dry.
But in general, try to write about your past experience, how those experiences shape who you are today, what you have learned, why you want to go to X University, what your research interests are, and what you hope to learn and how going to X University can help you do that.