GENERAL ANATOMY OF EMBRYO OF 12 MM. 



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Fig. 109. — Pig Embryo of 12.0 mm. Reconstruction from the Transverse Sections, Series 5. 

 The embryo has been drawn as if transparent, to show the form of its pharynx and the relations of the pharyn- 

 geal gill pouches to the grooves on the outer surface of the embryo. Cbl, Cerebellum. C.S, Cervical sinus. 

 Dien, Diencephalon. H, Cerebral hemisphere. Hy, Hypophysis, which arises as an evagination from the 

 oral cavity. Inf, Infundibular gland, which arises as a median evagination from the floor of the fore-brain. 

 l.gr. Lachrymal groove, w, Maxillary process. M.b, Mid-brain. Aldb, Mandibular process. Md.ob, 

 Medulla oblongata, na.ex. External nasal opening, ni, A thin epithelial plate separating the nasal pit 

 from the oral cavity ; by the rupture of this plate the internal nares is formed later. Oe, OEsophagus. Op, 

 Eye. Ot, Otocyst. par, Part of the thymus, arising from the epithelium of the fourth gill cleft. Sp.c, 

 Spinal cord. T/i, Median thyroid gland, thym, Thymus. Tra, Trachea. /, Entodermal pouch of the 

 first branchial cleft, the anlage of the Eustachian tube and tympanum. 2, Entodermal pouch of the second 

 gill cleft; it actually opens to the exterior, j, Entodermal pouch of the third branchial cleft. 4, Ento- 

 dermal pouch of the fourth gill cleft, the lower fork of which is the anlage of the lateral thyroid. /, Ecto- 

 dermal groove of the first branchial cleft ; it is the anlage of the meatus auditorius externus. X 20 diams. 

 — [Drawn by Dr. F. T. Lewis.) 



