Fig. 4. D2-40 immunohistochemistry of lymphatic vessels at the lung surface. (A) The right lung base from a specimen at 9 weeks (the same specimen as shown in Fig. 3A; 39 mm crown-rump length [CRL]). (B) The right lung base from a specimen at 16 weeks (a mixture of the pseudoglandular and canalicular phases; 125 mm CRL). D2-40–positive lymphatic vessels reach the lung surface until 9 weeks (arrows in A) and they are thick and abundant at 16 weeks (arrows in B). Bronchial epithelia also express reactivity of D2-40. Veins reach the lung surface at 16 weeks. Two panels were prepared at the same magnification. Scale bar=1 mm (A).