MALFORMATIONS 



209 



extent to which the primitive trace is cleft, and also by the limita- 

 tions of the dichotomy to the cephalic or caudal extremity of the 

 neural axis. Either or both extremities may become bifid. The cepha- 

 lic or head extremity may become bifid alone and a double head, or 

 still further bifid and the posterior extremities single or the pos- 

 terior extremity become bifid and the anterior single. 



Figs. 72 and 73 illustrate a duplication of the legs. The rudimen- 

 tary legs are perfect, but not so well developed as the other two. 

 This is polymelus. 



UMBILICAL HERNIA OF THE BABY CHICK 



We have examined many chicks in cases where they have 

 died after partly pipping out of the shell. The shells were 



^ 



Fig. 74. Umbilical Hernia of a Baby Chick. 

 1, portion of egg sac not included; 2, umbilical ring. 



carefully removed and an examination, in many, showed a 

 rupture of the umbilicus at its point of attachment. At this 

 point the abdominal wall is very thin, and is not reinforced 

 by elastic tissue, as in (luadrupeds, and if the membranes 

 become very dry and tout>h, the straining- ))y the chick in its 

 efforts to free itself may prove fatal. Fig. 74 shows a 

 hernia a.s a result of excessive straining. No. 1 is the ab- 

 dominal yolk sac which has been forced thi'oua'h tht^ umbilicus, 

 Xo. 2. 



