At the end of 5th week of gestation, how many numbers of somites can be seen?

**Core Concept**
Somitogenesis is the process of somite formation in the developing embryo. Somites are paired blocks of mesoderm that give rise to various structures such as skeletal muscles, bones, and dermis.

**Why the Correct Answer is Right**
By the 5th week of gestation, the embryo has undergone significant somitogenesis. The paraxial mesoderm, located on either side of the notochord, begins to segment and form somites. This process is mediated by the segmentation clock, a complex genetic mechanism involving the expression of Hox genes and other transcription factors. The correct number of somites seen at this stage is 30-40 pairs.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because the number of somites is not 10-20 pairs at the 5th week of gestation.
**Option B:** This option is incorrect because the number of somites is not 50-60 pairs at the 5th week of gestation.
**Option C:** This option is incorrect because the number of somites is not 20-30 pairs at the 5th week of gestation.

**Clinical Pearl / High-Yield Fact**
The number of somites present at the 5th week of gestation serves as a useful landmark in embryological development. Abnormalities in somitogenesis can lead to various congenital anomalies, including vertebral and limb malformations.

**Correct Answer:** C. 30-40 pairs.