82 DIFFERENTIATION III 



determines the symmetry of the embryo, since the micromere 

 pole becomes the blastopore pole, and the original egg axis the 

 gastrula axis, or as nearly so as possible. The pigment is found 

 in any part of the larva, right or left, dorsal or ventral, anterior 

 or posterior. It is not therefore essential to development. It 

 may be added that the yellow pigment band of Strongy- 

 locentrotus is equally unnecessary. Normally it is subequa- 

 torial and passes into the archenteron, but it may be meridional 

 or oblique to the egg-axis, and so become incorporated wholly 

 or partly in the ectoderm (Garbowski). 



Experiments of a like kind on other eggs have yielded like 

 results ; for while the existence of an invisible structure has 

 been revealed, a structure which is not disturbed by the centri- 

 fuge and is definitely related to the subsequent differentiation, 

 that differentiation has been shown to be independent of the 

 distribution of some at least of the visible constituents of 

 the cytoplasm. 



Thus Lillie, by centrifuging the egg of Ghaetopterus during 

 the first maturation division, produced in it three layers : 

 a small grey cap at the centripetal pole, a clear layer, and 

 a yellow granular hemisphere (on the centrifugal side). These 

 strata, it was found, might occupy any position with regard 

 to the egg-axis (as defined by the polar bodies), yet in 

 fertilization the sperm always entered at the vegetative pole, 

 and cleavage was always normally related to that axis. The 

 grey cap is derived from the contents of the germinal vesicle, 

 the clear band from the mierosomes of the endoplasm, and the 

 yellow granules from the coarser endoplasmic constituents. 

 It would be interesting to know the further history of these 

 centrifuged eggs. 



This we do know in other cases. 



The ovum of the Lamellibranch Cumingia contains a red 

 pigment and an oily green material both scattered through 

 the cytoplasm. When the egg is centrifuged during the 

 first polar division (Morgan) these go to opposite poles, the 

 red pigment to the centrifugal, the green oil to the centri- 

 petal. Between the two is a broad hyaline layer. Maturation 

 proceeds and the polar bodies are extruded. 



