SECT. l] 



PHYSICO-CHEMICAL SYSTEM 



347 



McClendon extended his observations to the egg of the sea-urchin, 

 Arbacia punctulata. Separated by centrifugal force, this egg divided 

 itself into four layers, as Lyon had already described, {a) a layer of 

 yolk bodies and red pigment granules extending from the centrifugal 

 end about half-way to the equator, {b) a layer of similar yolk bodies 

 but without the pigment granules, {c) a translucent fluid layer ex- 

 tending almost to the centripetal pole and containing the nucleus, 

 and finally {d) a very opaque layer or cap of minute volume, sitting 

 on the centripetal pole. When the crushed eggs were centrifuged, 

 the material separated into two layers, {a) and {b) being indistin- 

 guishable, centrifugal and containing the egg-membranes, and 

 [c) centripetal, {d) not being perceptible. McClendon analysed the 

 layers in the same manner as those of the frog's egg — the figures 

 are given in Table 43. 



Table 43. 

 McClendon' s figures (1909). 



A short consideration of them shows that centrifugal force is not 

 nearly so successful in separating the egg of the sea-urchin into 

 chemically unlike layers as it is in the case of the frog. This fits in 

 perhaps with the long-established fact that centrifugal force inter- 

 feres far less with normal development in the sea-urchin's egg than 

 it does in the frog's egg (Morgan and Lyon). It was very noticeable 

 that, whereas the frog's egg separated out into layers of markedly 

 different water-content, this did not take place in the sea-urchin's 

 egg. In the case of the centrifuged frog's egg, again, there were big 

 differences between the phosphorus contents of the different layers, 

 but in that of the sea-urchin's egg this only applied to the residues 

 which were mainly protein. McClendon surmised that the inclusion 



