68 



THE ANCESTRY OF VERTEBRATES 



FRORIEP however, declares that all the somites are metotic 

 or occipital and that prootic somites do not exist (l.c.p. 37)> 

 If former investigators believed that they had observed them, 

 this is caused by the erroneous assumption that the auditory 

 vesicle has a fixed situation in regard to somites and visceral 

 clefts. In reality, however, it wanders backwards during 



Fig. 16 Embryo of Torpedo ocellata, stage D. 

 aud auditory vesicle, Ch rostral end of 

 the notochord, n-z occipital somites 

 (after Froriep, 1902). 



development, while at the same time the foremost somites 

 undergo a reduction, being dissolved into mesenchyme. 

 At the same time the originally insignificant head-mesoderm 

 spreads over the branchial region. It occupies dorsallythe 

 place left by the dissolved somites and seizes upon the 

 part of the notochord which was formerly flanked by the 

 latter and at both sides of which will appear the parachor- 

 dalia, which accordingly belong to the praespinal region. 

 The fixed point, from which to establish all these dislocations, 



