PRAEORAL LOBE. 279 



THE PR.-EORAL LOBE IN THE CRANIATE VERTEBRATES. 



After what has been said above, in this and the preced- 

 ing chapters, the question as to how the praeoral lobe is 

 represented in the craniate Vertebrates need not detain us 

 long. 



Since, as shown above, the nervous element of the prae- 

 oral lobe (apical plate and cerebral ganglion) is entirely 

 lacking in the Vertebrates, we can only expect to find the 

 mesodermal element represented in the head-cavities of 

 the higher forms. 



In consequence of the great development of the brain, 

 even in the lowest craniate Vertebrates, as compared with 

 Amphioxus, and in consequence too of the cranial flexure, 

 the head-cavities have been made to assume a more sub- 

 ordinate position, and no longer take part in the formation 

 of a prominent lobe in front of the body. This is a perfect 

 illustration of "le principe du balancement des organes " 

 of Geoffroy Saint-Hilaire, the praeoral lobe decreasing as 

 the brain increases. A comparison between Figs. 70, 

 72, 117, and 135 will show at once that the praeoral head- 

 cavities of Amphioxus and Balanoglossus are the homo- 

 logues of the pr&mandibular head-cavities of the craniate 

 Vertebrates. 



These cavities lie at first below the mid-brain, and later 

 their walls give rise to most of the eye-muscles. In Figs. 

 91 and 135 the median portion of the praemandibular 

 cavities can be seen still in the form of an anterior pocket 

 of the endoderm, and it may be noticed how far it is 

 removed from the anterior extremity of the body to which 

 it extends in Amphioxus, etc. In the craniate Verte- 

 brates the brain extends forwards, and the head-cavities 



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