290 



EMBRYOLOGY. 



of growth by comparing fig. 154 with fig. 156 and fig. 157 with 

 fig. 158. 



The development of the visceral clefts and the cervical sinus has also a 

 practical interest. Sometimes there occur in the neck-region in Man fistulse, 

 which penetrate variable distances from without inward, and may even open 

 into the pharyngeal cavity. They result from embryonic conditions, the 

 cervical sinus having remained partly open. From this sinus a passage may 



Fig. 157. Very young human embryo of the fourth week 4 mm. long, neck-rump measurement; 

 taken from the uterus of a suicide 8 hours after her death, after RABL. 



au, Eye ; ng, nasal pit; ulc, lower jaw ; zb, hyoid arch; s s , s*, third and fourth visceral arches; 

 h, protrusion of the wall of the trunk produced by the growth of the heart ; us, boundary 

 between two primitive segments ; oe, ue, anterior and posterior limbs. 



lead, even in the adult, into the pharyngeal cavity, if abnormally the second 

 visceral cleft has not closed. 



C. The Development of the Anus and the Post-anal Gut. 



The question concerning the fate of the primitive mouth [blastopore] 

 and the development of the anus is not yet settled. Many disclosures 

 are still to be expected from a comparative study of these structures 

 in the different classes of Vertebrates. According to the common 

 representation, which appears to me to correspond on the whole 

 with the real state of affairs, the primitive mouth is a transitory 

 structure without permanent existence. In all Vertebrates it is 

 surrounded, as in Amphioxus, by the growth of the medullary folds, 



