II CLEAVAGE 69 



both cases in the D quadrant, may readily in one case pass 

 into the cell 2 d, in another into 4 d, and is then dubbed 

 ectodermal in the one case, mesodermal in the other. It is 

 none the less the same organ, homologous in the two forms, 

 since its homology depends eventually on the conditioning 

 factor, the cytoplasmic substance which is specific for it, and 

 not, as has been sometimes supposed, on the cell into which 

 that substance happens to pass. 



And fourthly, it is possible to suppress cleavage without 

 preventing differentiation. At least Lillie has been able, by 

 the use of potassium chloride, to show that the unsegmented 

 egg of Chaeto2^teni8 will put out cilia, while certain cyto- 

 plasmic substances undergo a change of position. In particular 

 certain granules assume the position normally taken by them 

 in the cells of the prototroch. It is interesting to note that in 

 these ova the nucleus enlarges, so that the plasma-nucleus 

 ratio is reduced. 



But though the factors on which the pattern and symmetry 

 of segmentation depend are distinct from those which condi- 

 tion the symmetry of the embryo, it must not be forgotten 

 that they may coincide. Thus we have seen that in the 

 Frog's egg, the plane of symmetry, the first furrow, and the 

 sagittal plane may sometimes all be coincident, and there are 

 other cases. In Amphioxus (Cerfontaine) and in Ascidians 

 (Conklin) this occurs and again in the Cephalopod Mollusca, 

 though it remains for a statistical examination to show that 



o 



the coincidence is invariable. In spiral eggs also, the median 

 plane of the embryo usually has a definite relation to the 

 cleavage system, the D quadrant being as a rule posterior. 



Experimental analysis, however, shows the difference. The 

 causes which determine a particular pattern of cleavage, as we 

 know, consist in the physical properties of the cytoplasm, 

 and in the relation between the cytoplasm with its con- 

 stituents and the dividing nuclei with their centrosomes. 

 The causes of differentiation on the other hand, as we are 

 now to see more fully, reside in the first instance in specific 

 organ-forming cytoplasmic materials. 



