Hi Mary,

Where the stem was attached to the tomato is called the stem scar, the cracks emanating from the stem scar are referred to as side cracks or radial cracks. The cracks are due to excessive moisture or heat along with the rapid growth of the fruit causing the skin to crack.  Some varieties are more prone to cracking. Check out the following links:

Vegetable: Tomato, Fruit Cracking | Center for Agriculture, Food, and the Environment at UMass Amherst

What Causes Tomatoes to Crack? | N.C. Cooperative Extension (ncsu.edu)

I hope this helps.