8 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 56 



ventrad as a narrow, solid tongue of epithelium towards the exterior, 

 figure 51, and fuses with the superficial ectoderm at the caudal end 

 of a prominent ridge that lies in the m.id-ventral line between the 

 posterior appendages. In this embryo the cloaca has no actual open- 

 ing to the exterior ; the walls of the part that projects towards the 

 exterior are in close contact, except in the region of the openings of 

 the Wolffian ducts, as is shown in figure 5H. 



Owing to the coiling of the end of the long tail the plane of the 

 section, as is seen in figure 51, passes through the posterior end of 

 the embryo no less than four times. In the most posterior of these 

 four sections of the tail, beginning slightly caudad to the section 

 here shown, is seen a small cavity which may be called the post- 

 anal gut, pag. It has thick walls, and extends for about thirty-five 

 sections in the series under discussion. Its lumen is very large in its 

 caudal region, figure 51, pag, and tapers gradually cephalad until it 

 disappears. Posteriorly the post-anal gut ends quite abruptly not 

 very far from the extreme tip of the tail. 



Figure 5J is a composite drawing from reconstructions of the 

 enterons of two embryos of approximately this stage. One of these 

 reconstructions was plotted on paper from a series of transverse 

 sections ; the other was made in wax from a series of sagittal sec- 

 tions. For the sake of simplicity the gill clefts are not represented, 

 and the pharynx, mouth, and liver are represented in outline only. 

 For the same reason the lung rudiment of one side only is shown. 



The relative size of the pharynx, ph, as seen in the figure, is 

 smaller than it is in reality because of the small dorso-ventral diam- 

 eter (the only one here shown) compared to the lateral diameter. 

 The end of the lung rudiment. In, is slightly enlarged and lies in 

 a plane nearer to the observer than that of the oesophagus, oe, though 

 this is not well shown in the figure. 



The oesophagus, oe, diminishes slightly in caliber for a short 

 distance caudad to the origin of the lungs, then gradually increases 

 in caliber until it suddenly bends to the side (towards the observer) 

 and merges into the wide stomach, i'. The stomach, which is irregu- 

 larly conical in shape, lies in a place slightly nearer the observer than 

 the end of the lung rudiment mentioned above. 



Lying to one side of the stomach and duodenum, and extending 

 cephalad beyond th-e end of the lung rudiment is the liver, li, whose 

 outline is only roughly shown here by the broken line. The stomach 

 opens rather abruptly into the duodenum, d, which slopes back 

 towards the plane of the oesophagus (away from the observer). 



The projection from the side of the duodenum, pan, not well 

 figured here, indicates the position of the pancreas, better shown in 



