. A laboratory manual and text-book of embryology. Embryology. 120 THE STUDY OF SIX AND TEN MILLIMETER PIG EMBRYOS Section through the Anlages of the Metonephroi, Cloaca and Hind-gut (Fig. 112).—The metonephroi are seen as dorsal evaginations from the mesonephric (Wolffian) ducts just before their entrance into the cloaca. Each consists of an epithelial layer surrounded by a condensation of mesenchyme. Traced a few sections cephalad the mesonephric ducts open into the lateral diverticula of the cloaca, which, irregular in outline, because it is sectioned obliquely, lies ventral to them and rec
. A laboratory manual and text-book of embryology. Embryology. 120 THE STUDY OF SIX AND TEN MILLIMETER PIG EMBRYOS Section through the Anlages of the Metonephroi, Cloaca and Hind-gut (Fig. 112).—The metonephroi are seen as dorsal evaginations from the mesonephric (Wolffian) ducts just before their entrance into the cloaca. Each consists of an epithelial layer surrounded by a condensation of mesenchyme. Traced a few sections cephalad the mesonephric ducts open into the lateral diverticula of the cloaca, which, irregular in outline, because it is sectioned obliquely, lies ventral to them and receives dorsad the hind-gut. Caudal to the cloaca in this embryo, the tail bends abruptly cephalad and to the right. The blind prolongation of the hind-gut may be traced out into this portion of the tail until it ends in a sac-like dilatation. n. umbilical vein artery Tall Mesonephric duct Metonephric onlag Spinal cord. Ventral body wall _Lumbilical artery L umbilical vein Allantoic stalk Cloaca Hind-gut Notochord Fig. 112.—Transverse section through the anlages of the metanephroi of a six mm. pig embryo. X 45. B. THE ANATOMY OF TEN TO TWELVE MM. PIG EMBRYOS The study of embryos at this stage is important as they possess the anlages of most of the organs. The anatomy of a 12 mm. pig embryo has been carefully studied and described by Lewis (Amer. Jour. Anal., vol. 2, pp. 211-225, I9°3)- External Form (Fig. 113).—The head is now relatively large on account of the increased size of the brain. The third branchial arch is still visible in the embryo, but the fourth arch has sunken in the cervical sinus; usually both have disappeared at a slightly later stage. The olfactory pits form elongated grooves on the under surface of the head and the lens of the eye lies beneath the ectoderm surrounded by the optic cup. The maxillary and mandibular processes of the first branchial arch are larger and the former shows signs of fusing with the median nasal process to form the u
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