190 REPRODUCTION 



Other observers have been able to maintain a single race 

 of Paramoecia for over five years and carry it through three 

 thousand generations by simple fission without conjugation 

 having occurred. This was accomplished by continually 

 altering the character of their food and as far as possible 

 imitating the conditions of pond-life. Another culture which 

 is, I believe, still "going strong" was started some fifteen or 

 more years ago. 



One of the ductless glands of the Mammalia, the pituitary 

 body, situate at the base of the brain, is associated with 

 sexual changes, and by administering extracts of this gland to 

 Paramoecia reproduction was markedly increased. This is 

 a remarkable fact, for the Paramoecium is as widely removed 

 from the mammal as any animal could well be. The secretions 

 of the pituitary body which pass into the blood also have 

 a great effect on the growth of the body. 



Artificial parthenogenesis has been known since the end of 

 the last century. Eggs of the silk-worm moth and of the 

 marine worm, Chaetopterus, can be induced to show the early 

 signs of fertilization by the addition of certain chemicals. 

 By concentrating the amount of salts it was possible to induce 

 the unfertilized eggs of the sea-urchin to develop as far as 

 the larval stage, termed the Pluteus. The rate of development 

 was slower than when the ovum had been fertilized by a 

 spermatozoon and there were other irregularities; but still 

 the larvae were there. Similar experiments succeeded more 

 or less with the eggs of certain limpets and with those of the 

 lamprey and of the frog. 



Unfertilized eggs may be induced to develop not only by 

 the application of certain salts and the variation of the media 

 in which they live, but by electrical and mechanical stimuli. 



Gently brushing with a camel's-hair brush or applying 

 electric discharges to the egg, even by submitting the ovum 

 to pin-pricks, will start the unfertilized egg segmenting, but, 

 as a rule, it does not go very far. Occasionally larvae and 

 even adults have been reared as the result of these mechani- 

 cal processes. A young sea-urchin has been raised from an 

 unfertilized egg, and by pricking frogs' eggs with a platinum 

 needle, which was sometimes dipped in salt or in the blood of 



