500 EMBEYOLOGY OF THE LOWER VERTEBRATES oh. 



(Nauplius, Trochosphere) is practically a precociously developed and 

 free-living head which has not yet developed a trunk. 



As will have been gathered, one of the most conspicuous features 

 of the head-region is the loss of segmentation in organs in which it 

 was once present. 



Metameric segmentation, which first makes its appearance in 

 typical form in the Annelida, is probably to be associated primarily 

 with the coelome and its lining the mesoderm. The coelome is 

 distended with coelomic fluid and the turgidity so caused gives 

 firmness to the body. The physiological advantage of the coelomic 

 cavity being subdivided into successive compartments is obvious. 

 The segmentation of other organs is to be looked on as secondary 

 to that of the mesoderm, and more especially to that of the 

 muscles. Thus the segmented character of the nervous system of 

 an Annelid or Arthropod is due to the ganglion-cells tending to 

 become concentrated at the level of the masses of muscle 

 which work the parapodia or limbs. So also the segmentation of 

 the skeleton which permits flexure of the body is correlated directly 

 with the segmentation of the musculature which causes that 

 flexure. 



So, conversely, with the disappearance of segmentation in the 

 head of the Vertebrate. Correlated with the loss of flexibility in 

 the brain region the myotomes which produce the flexure have 

 disappeared, and correlated with this in turn the ensheathing 

 skeleton has lost its segmentation and the segmentally arranged 

 motor nerves have also gone. The process has taken place from 

 before backwards. It has been carried to the greatest extent in 

 front, to the least at the hinder limit of the head. 



It is definitely established that the head of the Vertebrate 

 has at least in part come into being by the modification of what 

 was once the anterior portion of the trunk. With the gradual 

 evolution and increase in size of the brain — so characteristic of the 

 phylum Vertebrata — this organ has gradually encroached upon the 

 spinal cord, and its protective skeleton the chondrocranium has 

 pari passu encroached upon the vertebral column. This is clearly 

 indicated by the fact that included within the limits of the skull 

 are nerves which are serially homologous with those of the trunk. 

 Putting on one side the probability — as many would regard it — 

 that cranial nerves III, IV, V, VI, VII, IX and X are really 

 homologous with the spinal nerves, we find behind the Vagus a 

 series of spino-occipital nerves (Fiirbringer, 1897), which although 

 included within the limits of the skull are yet undoubtedly members 

 of the same series as the spinal nerves. The number of those is 

 very different in the different subdivisions of the Vertebrata as may 

 be gathered from an inspection of Fig. 222. In all probability they 

 will be found also to show considerable variation in different 

 individuals of the same species. 



During the evolution of the head there is some reason to believe 



