III. 7 OSMOTIC PRESSURE 123 



the solutions, though what this is is not known. 1 It may be 

 added that in Gurwitsch's experiments the concentrations of the 

 alkaloids employed were certainly far below those which would 

 be isotonic with a -625 % solution of sodium chloride. It also 

 follows that during the closure of the blastopore the Frog's egg 

 does not need to absorb water from the outside ; it may, in fact, 

 be exposed to a very considerable degree of desiccation at this 

 period without interfering in the least with the closure of the 

 blastopore or of the medullary folds, a result which is all the 

 more surprising in that the newly hatched tadpole imbibes water 

 at so rapid a rate. 



The experiments which have hitherto been considered relate 

 to the need of water for normal development. There are, 

 however, certain processes for which not the absorption, but, 

 on the contrary, the abstraction, or at least the local abstraction, 

 of water appears to be essential, the phenomena, namely, of 

 fertilization. Cytologists have observed that the entrance cone 

 and funnel, the mechanism by which the spermatozoon is swept 

 into the interior of the egg, appear to be aggregations of 

 a watery substance about the acrosome or apical body, and that 

 the sperm sphere and aster are similarly due to the withdrawal 

 of water by the centrosome in the middle-piece from the cyto- 

 plasm; in other words, that the stimulus whereby the sperma- 

 tozoon restores to the egg its lost power of cell-division is 

 essentially a process of local dehydration. 



This inference is substantiated by the familiar experiments 

 of Loeb, who has succeeded in rearing normal larvae from the 

 unfertilized eggs of Echinoderms and certain worms by tem- 

 porary immersion in certain solutions. In his earlier experiments 

 he found that a mixture in equal parts of a 2 g n solution of 

 magnesium chloride and sea-water produced more Plutei than 

 any other solution tried, and hence believed the result to be 

 specific and attributable to the magnesium ion. Later, however, 

 this artificial parthenogenesis was successfully brought about by 

 various isotonic solutions (chlorides of sodium, potassium and 

 calcium, potassium bromide, nitrate and sulphate, cane-sugar 



1 In this view Stockard, as a result of experiments on Fundulus, concurs 

 (Arch. Ent. Mech. xxiii, 1907, and Jonm. Exp. Zool. iv, 1907). 



