DEVELOPMENT OF THE ALIMENTARY TUBE AND APPENDED ORGANS. 359 



frequent, apart from transposition, include anomalies in the size and number of 

 lobes. Accessory lobes may occur within the falciform ligament. One case 

 of lack of development of the gall bladder has been observed. Stenosis of the 

 bile passages is occasionally met with. 



THE PANCREAS. Occasionally accessory glands are found in the intesti- 

 nal or gastric wall. These probably represent aberrant portions of the main 

 gland, and may give rise to cystic structures. Very recently, however, a 

 number of intestinal diverticula have been observed in certain mammalian 

 embryos and also in human embryos.* Although the history of these unusual 

 diverticula has not been traced, their presence may offer a clue to the origin of 

 accessory pancreatic structures. The ducts of the pancreas are subject to 

 distinct variations, which, however, are not usually considered as anomalies. 

 Not infrequently the duct of the dorsal anlage (duct of Santorini) persists and 

 opens directly into the duodenum. It may persist along with the duct of the 

 ventral anlage (duct of Wirsung), or the latter may disappear (p. 353; compare 

 Figs. 316 and 317). 



PRACTICAL SUGGESTIONS. 



The Primitive Gut. The formation of the primitive gut can be studied in chick embryos 

 prepared according to the technic on page 385, under the head of the Ccelom and Common 

 Mesentery. 



The Mouth and Pharynx. The external appearance of the oral pit and the branchial 

 arches and grooves can be studied in very young pig embryos (6 mm. or less). The speci- 

 mens should be preserved in toto in 4 per cent, formalin, and for class purposes can be 

 mounted in glycerin jelly (see Appendix). Occasionally young human embryos (8 mm. 

 or less) are obtained, in which the branchial arches and grooves show clearly. 



For the study of the internal structure, sections of course are necessary. Pig embryos 

 afford very good material and are easily obtained. The specimens may be fixed in Zenker's 

 or Bouin's fluid, cut in celloidin or in paraffin, and stained with hsematoxylin and eosin. 

 Usually it is advisable to cut serial sections. The stages given in the following paragraphs 

 are perhaps the most convenient for the study of the different organs. 



The tongue can be seen in its earlier stages in sagittal sections of pig embryos of 12 to 

 18 mm. The development of the teeth can be followed in embryos of 20 to 100 mm. The 

 most convenient way is to remove the jaws, fix for several days in Bouin's fluid, which 

 at the same time decalcifies, and cut sections transversely to the long axes of the jaws. 

 Very beautiful preparations can be obtained in this way. The anlagen of the salivary glands 

 can be seen in embryos of 15 mm. The anlagen of the thyreoid and thymus are very clearly 

 shown in embryos of 10 to 15 mm. 



The oesophagus can be seen in any transverse section in the thoracic region of an embryo 

 of any stage after the formation of the primitive gut. 



The Stomach and Intestine. The development of these organs can be studied in transverse 

 sections of pig embryos of from 6 mm. up to any size that is not too large to section. Zenker's 

 and Bouin's fluid are both good fixatives. It is best to cut transverse serial sections, 

 although sections taken at intervals are very instructive. Haematoxylin and eosin give a 



* Lewis.. Thyng. 



