260 THE FR.EORAL LOBE. 



and by the distribution of the vagus nerve in the Vertebrata\ On 

 the other hand the insertion of the liver, which was probably a very 

 primitive organ, appears to indicate with approximate certainty the 

 posterior limit of the branchial clefts. 



With these few preliminary observations we may pass to the 

 main subject of this section. A fundamental question which presents 

 itself on the threshold of our enquiries is the differentiation of the 

 head. 



In the Chsetopoda the head is formed of a prseoral lobe and of the 

 oral segment ; while in Arthropods a somewhat variable number of 

 segments are added behind to this primitive head, and form with it 

 what may be called a secondary compound head. It is fairly clear 

 that the section of the trunk, which, in Amphioxus, is perforated by 

 the visceral clefts, has become the head in the Vertebrates proper, 

 so that the latter forms are provided with a secondary head like 

 that of Arthropods. There remain however difficult questions (1) as 

 to the elements of which this head is composed, and (2) as to the 

 extent of its differentiation in the ancestors of the Chordata. 



In Arthropods and Choetopods there is a very distinct element in 

 the head known as the procephalic lobe in the case of Arthropods, 

 and the prseoral lobe in that of Chaitopods ; and this lobe is especially 

 characterized by the fact that the supraoesophageal ganglia and optic 

 organs are formed as differentiations of part of the epiblast covering 

 it. Is such an element to be recognized in the head of the Chordata? 

 From a superficial examination of Amphioxus the answer would 

 undoubtedly be no; but then it has to be borne in mind that Amphi- 

 oxus, in correlation with its habit of burning itself in sand, is especially 

 degenerate in the development of its sense-organs ; so that it is not 

 difficult to believe that its prseoral lobe may have become so reduced 

 as not to be recognizable. In the true Vertebrata there is a portion 

 of the head which has undoubtedly many featiires of the prasoral 

 lobe in the types already alluded to, viz. the part containing the 

 cerebral hemispheres and the thalamencephalon. If there is any 

 part of the brain homologous with the supraoesophageal ganglia of 

 the Invertebrates, and it is difficult to believe there is not such a 

 part, it must be part of, or contain, the fore-brain. The fore-brain 

 resembles the supraoesophageal ganglia in being intimately connected 

 in its development with the optic organs, and in supplying with nerves 

 only organs of sense. Its connection with the olfactory organs is an 

 argument in the same direction. Even in Amphioxus there is a 

 small bulb at the end of the nervous tube supplying what is very 



1 The extension forwards in the Vertebrata of an uninterrupted body -cavity into 

 the region previously occupied by visceral clefts presents no difficulty. In Amphioxus 

 the true bod,> -cavity extends forwards, more or less divided by the branchial clefts, for 

 the whole length of the branchial region, and in embryos of tlie lower Vertebrata there 

 is a section of the body-cavity — the so-called head-cavities — between each pair of 

 pouches. On the disappearance of the pouches all these parts would naturally coalesce 

 into a continuous whole. 



