They can be distinguished by the location of the primary spermathecal pores in parietal invaginations. The maxillary sinuses form as invaginations from the nasal sac. The photosynthetic apparatus is housed in these invaginations. Sulfur is deposited in invaginations within the cell membrane. They arise five weeks after conception as bilateral invaginations of the walls. The wall of the large intestine is lined with simple columnar epithelium with invaginations. Some have even evolved cutaneous invaginations for aerial gas exchange. It was composed of spongy tissue due to many invaginations in the structure. Numerous intestinal glands as pocket-like invaginations are present in the underlying tissue. They also occur in epidermal invaginations of the plantar foot surface called rete ridges. These invaginations are also known as chromatophores. The inner surface of the colon is lined with simple columnar epithelium with invaginations. The epithelial inner surface of the colon is punctuated by invaginations, the colonic crypts. Crypts are anatomical structures that are narrow but deep invaginations into a larger structure. The invaginations are called intestinal glands or colon crypts.