1883.] Mr Sedgiuick, On the origin of segmented animals, etc. 61 



gives the ratio of the difference between theory and observation to 

 the whole relative retardation. Thus the general result I have 

 obtained is that MacCullagh's theory gives values of the relative 

 retardation which are slightly too large. The differences in the 

 last three or four rings may be fairly put down to errors of 

 observation. 



I have said nothing about the surface effects. If the plate had 

 been isotropic, the plane of polarisation would have been turned 

 a little when the light entered the crystal and again turned a little 

 in the same direction when it left it. If be the angle between 

 the plane of polarisation of the incident light and the plane of in- 

 cidence, and 6' be a similar angle with regard to the emergent 

 light, then tan & = tan 6 sec 2 (<£' - <f>). This gives & - 6 = 12' at 

 the most in the case of the 10th ring. Now it requires a rotation 

 of either Nicol through two right angles to move a dark band from 

 its place as one ring to its place as the next, so this small change 

 in 6 would have no appreciable influence. We may assume, I 

 think, that the surface effects in quartz are of something the same 

 nature and magnitude as in an isotropic substance. 



The experiments were made in the Cavendish laboratory in 

 August and October. I am much indebted to Mr Glazebrook for 

 his kind assistance and advice throughout. 



(4) On the origin of segmented animals and the relation of the 

 mouth and anus to the mouth of the Godenterata. By Adam 

 Sedgwick, M.A. 



In the discussion which followed the communication of the 

 late Professor Balfour's notes and drawings of the early embryos of 

 Peripatus Capensis to the Royal Society (December, 1882) I drew 

 attention to the great resemblance between the embryo of P. 

 Cap en sis with its elongated blastopore and somites, and an adult 

 Actinozooid Polyp. I pointed out that the comparison 

 of these two structures suggested an explanation which, so far 

 as I know, has not before been suggested, of a great morpho- 

 logical difficulty, viz. the origin of metameric segmentation (vide 

 Nature, Dec. 28, 1883). At the same time I pointed out that by 

 following up this comparison some other morphological difficulties 

 received an explanation. The hypotheses 1 I suggested were 

 shortly as follows. 



1 Mr E. B. Wilson who was present when this discussion took place at the Boyal 

 Society and to whom I subsequently at Cambridge showed the specimens and draw- 

 ings of the Peripatus embryo, informs me that the work (referring to Polyps) which 

 he has since done at Naples has enabled him to give some additional evidence in 

 favour of my views. As Mr Wilson's observations are not yet published, I am 



