484 'I'HOiMAS H. BRYCE. 



when it occurs. Thus the nucleus may divide repeatedly 

 without division of the cytoplasm^ and then the egg 

 may break into as many segments as there are nuclei 

 (Wilson and others). It is only in a relatively small pro- 

 portion of eggs that division is regular enough to permit of 

 development to the larval stage. Further, the eggs of the 

 same species behave capriciously to the same agents under 

 difPerent conditions (temperature^ etc.), and the eggs of closely 

 allied species seem to react differently to the same agent. 



There is no reasonable doubt, however, that true artificial 

 parthenogenetic development has been demonstrated for the 

 Echinoderms — sea-urchins and star-fish — and for at least two 

 Annelids, though the same amount of independent testimony 

 is not available for the latter. 



Actual development to a larval stage has been obtained 

 only by certain of the agents enumerated above. 



1. Increase of Osmotic Pressure. — The most successful 

 results (Loeb, 1902) are obtained at a temperature about 

 20° C, by the addition of the chlorides of potassium or sodium 

 to sea water, the optimum degree of concentration being 

 determined by experiment for each set of observations.^ 

 After the eggs have remained in this for half an hour to two 

 hours, the optimum being again tested by experiment, they 

 are restored to normal sea water. 



Sea-urchins (Loeb, Wilson, Giard, Prowazek, Delage, 

 Viguier, Hunter). Annelids : Chsetopterus, Amphitrite, 

 Nereis (Loeb, Fischer). 



2. Agitation. — Asterias (Mathews), Chtetopterus and 

 Amphitrite (Loeb, Fischer), but not sea-urchins (Mathews, 

 Vignier) . 



3. Elevation of Temperature. — Asterias during maturation 

 (Delage) ; not for ripe eggs (Greeley, Viguier). 



4. Depression of Temperature. — Asterias (Greeley) ; not 

 sea-urchins (Viguier). 



' Loeb (1902) uses a stock sohition of 2^ n. TICl, and adds tliis in difTeront 

 l)ropoitions, 8, 10, 12, 11, 10, 18 com., to 100 c.c. of sea water in six 

 vessels to dcteiniiiic the best iinxdc of concentration. 



