30 ILLINOIS ACADEMY OF SCIENCE 



Nearly all of the great American types of vegetation were 

 visited and studied, numerous photographs being taken and 

 numerous plant specimens being collected. Professor Henry 

 C. Cowles of the University of Chicago had general charge 

 of the excursion, but was ably assisted by Professor Frederick 

 E. Clements of the University of Minnesota; Dr. George E. 

 Nichols of Yale University, Dr. George D. Fuller of the Uni- 

 versity of Chicago, and Dr. Alfred Dachnowski of Ohio State 

 University, as well as by large numbers of botanists at the 

 places visited. 



RECENT THEORIES OF FERTILIZATION AND 

 PARTHENOGENESIS 



BY FRANK R. LILLIE, UNIVERSITY OF CHICAGO 



Two main problems of fertilization stand in the foreground 

 of all recent work viz : the problem of the initiation of develop- 

 ment and the problem of bi-parental inheritance. The latter 

 problem has received a much more complete and satisfactory 

 analysis than the former which is alone under consideration in 

 the present paper. The problem may be stated thus : Why is 

 the unfertilized egg usually incapable of development? And 

 how does the spermatozoon initiate the developmental pro- 

 cesses? 



This problem was naturally conceived in the early part of 

 the modern period (1875 to the present) in morphological 

 terms; and Boveri's theory that the egg did not develop be- 

 cause it lacked the "organ of cell-division" (viz : the cen 

 trosome) and that the spermatozoon enabled it to develop by 

 supplying the missing organ was long held by most natural- 

 ists. But with the advent of physiological analysis of cell life 

 the inadequacy of this point of view soon made itself felt. 

 The discovery of artificial parthenogenesis by Loeb started a 

 new era in this problem. It was shown that the role of the 

 spermatozoon could be dispensed with in initiation of de- 

 velopment, and that a change in the chemical constitution of 

 the medium could take its place. In such a case it was obvious 

 that no new cell organ was needed by the egg. Following this 

 epoch-making discovery a great many investigators studied 

 the problem of initiation of development in the eggs of various 

 species of animals, with the result that a great variety of 

 agents was shown to be effective in the production of par- 

 thenogenesis in different forms. These can be classified 

 under the head of chemical agents, osmotic changes, temper- 

 ature changes, radiation, mechanical shock and pricking. In 



