The layer of the alimentary canal that forms crypts in between the base of villi is the mucosa layer.

Mucosa Layer:
The mucosa layer is the innermost layer of the alimentary canal, which lines the lumen of the digestive tract. It is composed of three sublayers: epithelium, lamina propria, and muscularis mucosae. The epithelium layer is the outermost layer of the mucosa, which is in direct contact with the lumen of the digestive tract. The epithelium layer is specialized for various functions, including secretion, absorption, and protection. The lamina propria is the second layer of the mucosa layer, which is a thin layer of connective tissue that contains blood vessels, lymphatic vessels, and immune cells. The muscularis mucosae is the third layer of the mucosa layer, which is a thin layer of smooth muscle that contracts to move the epithelium layer.

Crypts in between the base of villi:
The mucosa layer of the small intestine is lined with finger-like projections called villi, which increase the surface area for absorption. Between the base of villi, there are small invaginations called crypts of Lieberkuhn, which extend down to the muscularis mucosae. The crypts of Lieberkuhn are lined with stem cells, which give rise to various types of cells, including absorptive cells, goblet cells, and Paneth cells. The absorptive cells are responsible for absorbing nutrients from the lumen of the digestive tract, whereas the goblet cells secrete mucus to protect the epithelium layer. The Paneth cells secrete antimicrobial peptides, which protect the epithelium layer from bacterial infection.

Conclusion:
Thus, the mucosa layer of the small intestine forms crypts in between the base of villi, which are lined with stem cells that give rise to various types of cells, including absorptive cells, goblet cells, and Paneth cells.