114 THE STUDY OF SIX AND TEN MILLIMETER PIG EMBRYOS 



ducts lies the hind-gut, dorsal to the cloaca. The tip of the tail is seen in section at the 

 left of the figure. 



Section through the Anlages of the Metanephroi, Cloaca and Hind-gut (Fig. 117). 

 The metanephroi 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 duels 

 open into the lateral diverticula of the cloaca, which, irregular in outline because it is sec- 

 tioned obliquely, lies ventral to them and receives dorsad the hind-gut. Cauda tol 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. 



R. umbilical vein 

 R. umbilical artery 



Tail 



Mesonephric duct 

 Metanephric anlage 



Spinal cord 



Ventral body wall 



L. umbilical artery 

 L. umbilical vein 

 Allantoic stalk 



Cloaca 

 Hind-gut 



Notochord 



FIG. 117. Transverse section through the anlages of the metanephroi of a 6 mm. pig embryo. 



X45- 



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. Anat., 

 vol. 2, 1903). 



External Form (Fig. 118). 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. 



