Answer and Explanation:
Nutritive gelatine was the first agent used to solidify a culture. With time, gelatin was replaced by agar, which has proven to be much better on many occasions. Gelatine has an animal origin (cartilage), while agar has a vegetable origin (algae).
The disadvantage of using gelatin is that it needs an incubation temperature of 20ºC or less to solidify. Many times this temperature is inferior to the optimal temperature needed by bacterias to grow. The optimal temperature of many Micrococcus species is about 25 and 30ºC. At these temperatures, gelatin would probably melt.
Another important aspect is that many organisms might consume the gelatine, metabolizing it. In nature, Micrococcus species might be found growing in products of animal origin.
