118 



EXTERNAL FACTORS 



III. 7 



chloride) nuclear division continues all the same, for when re- 

 turned to sea- water the eggs divided at once into as many cells 

 as had in the meantime been formed in the controls, a result 

 confirmed by Morgan. 



That the normal egg is in a condition of osmotic equilibrium 

 with the sea-water is further shown by its behaviour in sea- 

 water diluted to twice its volume; in this experiment the egg 



FIG. 61. Variations in the segmentation of Echinus microtiibercidatus 

 produced by dilution of the sea-water, a, tetrahedral four-cell stage ; 

 b, eight cells, three premature micromeres ; c, eight cells, two precocious 

 micromeres ; d, the same egg after the next division, the precocious micro- 

 meres have divided unequally, two normal micromeres have been formed. 

 (After Driesch, 1895.) 



(of Arbacia) absorbs water, swells and bursts its membrane and 

 so produces a large ex-ovate which may develop independently 

 of the rest of the ovum (Loeb) when replaced under ordinary 

 conditions (Fig. 60). Driesch has produced irregularities of 

 segmentation by the same means (Fig. 61). 



Although, therefore, it seems reasonable to suppose that in 

 the cases just quoted the observed effects really are due to the 



