TEXT-BOOK OF EMBRYOLOGY. 



Anomalies. 



THE LARYNX. The larynx may be excessively large or unusually small. 

 Occasionally the epiglottic cartilage consists of two pieces, indicating a failure 

 of the two anlagen to fuse (p. 364). Similar defects may occur in the other 

 cartilages that are derived from more than one anlage. The ventricle on either 

 side may be abnormally large with an exaggerated appendage (laryngeal 

 pouch). This condition resembles that in the anthropoid apes. 



THE TRACHEA. The trachea is sometimes absent, in which case the bronchi 

 arise immediately below the larynx, indicating a failure on the part of the 

 original tube to elongate. The trachea may be abnormally short. Rarely 

 there is a direct communication between the trachea and oesophagus, probably 

 due to an incomplete separation of the lung groove from the gut (p. 362). The 

 cartilaginous rings may vary in number as a result of abnormal splittings and 

 fusions. 



THE LUNGS. Rarely the eparterial bronchial ramus on the right side 

 arises as a branch of the trachea and not as a branch of the bronchus (p. 367). 

 This condition is normal in certain Mammals (ox, sheep). Rarely an eparterial 

 bronchial ramus is present on the left side, thus producing a third lobe for 

 the left lung. In some animals an eparterial ramus is normally present on 

 each side, the larger bronchial rami thus being bilaterally symmetrical. Varia- 

 tion in size and number of lobes is not infrequent. Supernumerary or acces- 

 sory lobes, formed either by evaginations from the original anlage or by in- 

 dependent evaginations from the gut, are met with in rare cases. 



Occasionally some portion of either lung is defective. The bronchial bud 

 that would normally give rise to the lung tissue in that region fails to develop 

 properly, and the result is a number of rami, without the ultimate terminations, 

 surrounded by vascular tissue. The rami may remain normal or may become 

 dilated and form large bronchial cysts. 



PRACTICAL SUGGESTIONS. 



The anlage of the respiratory system can be seen in chick embryos about the beginning 

 of the third day of incubation, or in young mammalian embryos (pig embryos of 6-8 mm.). 

 Fix in Zenker's fluid or in Benin's fluid, cut transverse sections in the cervical region and 

 stain with Weigert's haematoxylin and eosin. Time can be saved by staining in toto with 

 borax-carmin, but the differentiation is not so good. Either technic can be used in follow- 

 ing succeeding stages of development, so long as the embryos are not too large for con- 

 venience in cutting sections. The structure and relations of the developing pleura can also 

 be seen in these sections. In fact it is possible to use the same sets of sections for the 

 study of the heart, pericardium, lungs and pleura. 



When the embryos are too large to section in toto, remove the lungs and subject them 

 to the above technic. Very interesting comparisons can be made between sections of lung 

 tissue from a still-bom child and from one which has breathed, but died shortly after. 



