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 

 H.umbilical 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, I 9°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 upper jaw. Small tubercles, the anlages of the external 

 ear have developed about the first branchial cleft which itself becomes the 

 external auditory meatus. 



At the cervical bend the head is flexed at right angles with the body bringing 

 the ventral surface of the head close to that of the trunk and it is probably owing 

 to this flexure that the third and fourth branchial arches buckle inward to form 

 the cervical sinus. Dorsad the trunk forms a long curve more marked opposite 



