138 THE THIRD DAY. [CHAP. 



stalk divides into two bands or nerves. Of these one passing 

 towards the eye terminates at present in the immediate 

 neighbourhood of that organ. Compare Fig. 46. The other 

 branch (the rudiment of the inferior maxillary branch of the 

 fifth nerve) is distributed to the first visceral arch (Fig. 46). 



The second mass (Fig. 25, VII. 45, VII.) is the rudiment 

 of the seventh, or facial nerve. It is the nerve of the second 

 visceral arch. 



The two masses behind the auditory vesicle represent 

 the glossopharyngeal and pneumogastric nerves (Fig. 45, VIII., 

 Fig. 46, G. Ph. and Pg.). At first united, they subsequently 

 become separate. The glossopharyngeal supplies the third 

 arch, and the pneumogastric the fourth arch. 



These four masses, representing four important mixed cranial nerves, 

 seem to be derived directly from the mesoblast surrounding the hind-brain. 

 It is worthy of notice that they are mixed, sensory and motor, nerves ; for, 

 restricted as are the sensory functions of the seventh, and the motor func- 

 tions of the pneumogastric in the adult mammal, the study of their com- 

 parative physiology leaves no doubt as to the essentially mixed nature of each. 

 It is also worthy of note that of the third, fourth and sixth nerves, no such early 

 rudiments appear; and there are reasons for thinking that these are in reality 

 intercranial branches, the third and fourth of the fifth, and the sixth of the 

 seventh nerves. The purely sensory nerve or rather sense-nerve, the auditory, 

 seems to have a different origin altogether from all the above, though it may 

 perhaps be looked upon as the dorsal branch of the seventh, while the erratic 

 hypoglossal appears to be distinctly a spinal nerve. 



Of the interesting relations of these cranial nerves to the visceral arches, we 

 shall have to speak more fully in the second part of this work, when we describe 

 the more primitive forms in the lower vertebrata. 



At the same time that these ganglia make their appearance, or a little earlier, 

 near the beginning of the third or end of the second day, there may be seen in 

 the region of the hind-brain, lines which appear to divide off the mesoblast on 

 either side into masses somewhat resembling protovertebrae. Of these masses 

 there are four or five on each side, generally three in front of, and two behind 

 the optic vesicle. They were first noticed by Remak, and are easily dis- 

 tinguished from the rudiments of the cranial nerves. They at first sight suggest 

 the idea of an initial and transitory segmentation of the cranial mesoblast into 

 protovertebrae. It seems possible that they are, in reality, appearances pro- 

 duced by a series of very characteristic transverse wrinkles into which the walls 

 of the hind-brain are at this time thrown, and which subsequently disap- 

 pearing altogether, as the walls increase in thickness, may perhaps be viewed 

 as indications of an aborted segmentation of the hind- brain into a series of 

 vesicles. The true nature of these quadrate masses is still very problematical. 



26. On the second day the newly formed Wolffian duct 

 extended along the greater part of the length of the embryo 

 as a tube resting on the mass of cells ,which we have already 

 called the intermediate cell-mass. 



