so 



ATLAS OF BIOLOGY. 



/./. Yolk-hearing lower-layer cells. 



mil. Mandibnlar arch. 



m.f. Medullary fold. 



m.ij. Medullary groove. 



.//. Muscle plate. 



ms. Undifferentiated mesoblast. 



na. Nasal involution (anterior nostril). 



n.n. Neural (cerebro-spinal) axis. 



nc. Nucleus. 



n.f. Neural caiml. 



n.e. Neurenteric canal. 



a. (EsophagUH. 



op. Optic cup (retina). 



op'. Optic stalk (optic nerve). 



]>d. Proctodoeum. 



jul'. Aperture of involution of the same (cloacal 



aperture). 



)il. Pulmonary sac (lung). 



jiji. Oral papillae. 



re. Prorenal (segmental) duct. 



s. Head suckers. 



sil. Stomodfeum. 



W. Aperture of involution of the same (oral 



aperture). 



XHI. Somatic mesoblast. 



xj>. Splanchnic mesoblast. 



ty. Tympano Eustachian cleft. 



t;/'. Membrana tympani. 



r.f. Posterior cardinal vein. 



