312 THE STUDY OF ANIMAL LIFE CHAP, xiv 



of the egg-cell a-dividing can be induced in many cases 

 by a great variety of stimuli. Such a development, 

 without the aid of a spermatozoon is described as 

 " artificial parthenogenesis." The stimuli may be 

 mechanical, chemical, thermal, electrical, and they are 

 known to work effectively in a great variety of cases,- 

 some starfishes, sea-urchins, worms, insects, molluscs, 

 fishes, and amphibians. It may serve to leave sea- 

 urchin ova for a short time in sea-water with slightly 

 altered composition and concentration, or to prick frogs' 

 eggs with a platinum needle and wash them in blood. 

 If development is to proceed normally and both sea- 

 urchins and frogs have been reared from artificially 

 parthenogenetic ova two steps seem to be necessary. 

 The egg-cell must be activated (by a positive stimulus 

 or by the removal of some obstacle to division) and 

 there must be an immediate corrective or counteractive 

 of this, otherwise the segmentation of the egg will simply 

 end in disintegration. 



