It's just a "mass of cells" at 24-weeks.
Thanks to advances in medical science, a baby born at 24 weeks has a higher chance of survival than ever before. The Life of a Baby Born at 24 Weeks
Twenty-four weeks puts your baby close to the end of your second trimester. By now, all your baby's organs are intact, and, basically, this time is being used for critical stages in development. Organs such as the brain are growing, the lungs are still developing, and there are many vital processes taking place regarding the maturity of your infant's body. Hence, when a baby is born at 24 weeks gestation, it is a huge cause for alarm. Most babies at this point barely weigh one and a half pounds. In this case, when a mother goes into preterm labor at this point in the second trimester, if the infant survives the birthing process (very often a cesarean is required at this gestational stage), he or she will be shipped off to the neonatal unit immediately. A baby born at 24 weeks will require a respirator immediately, as the lungs are not yet equipped to handle breathing outside the womb.
You can also expect surgeries due to insufficiently developed organs causing problems in your baby's body. Your infant will likely spend months under neonatal supervision, and it is possible that certain long-term health problems will arise due to this preterm labor. However, these health issues are not necessarily a given. Some babies born prematurely survive the process after a long NICU stay and emerge healthy overall, but it is normal to expect that the infant to toddler years may involve more concern and care.