4 ARTIFICIAL PARTHENOGENESIS AND FERTILIZATION 



unfertilized egg is unable to develop because it lacks the " organ" 

 for cell division. According to Boveri, this organ is the centro- 

 some, and it is introduced into the egg by the "middle piece" 

 of the spermatozoon. During nuclear division there appear 

 in the egg radiating figures, the astrospheres, whose physical 

 nature is at present unknown. Some eggs, but by no means 

 all, show at the center of a radiating system a granule, the 

 centrosome, and in this Boveri sees the "organ of cell division." 

 According to him the unfertilized egg lacks this granule which is 

 introduced at fertilization by the spermatozoon. 



Boveri's view that the centrosomes or astrospheres cannot 

 be formed by the unfertilized egg was disproved by Morgan, 

 who showed, as a sequel to my experiments on the effects of 

 hypertonic solutions upon the egg, that such treatment can 

 lead to the formation of astrospheres in the unfertilized egg, 

 and that these eggs can even begin to segment. 



In certain eggs, e.g. that of Chaetopterus, the spermatozoon 

 has to accomplish another task aside from fertilizing the egg, 

 namely, it must bring about the so-called maturation of the 

 egg. This consists in the reduction of the nucleus by two 

 subsequent nuclear divisions and the throwing-out of two frag- 

 ments of the nucleus, the so-called polar bodies. For the 

 maturation division, centrosomes and astrospheres are also 

 required. Mead showed that in the egg of Chaetopterus 

 the two astrospheres for the maturation division are present in 

 the egg before the spermatozoon enters it, but that this matura- 

 tion division cannot take place unless a spermatozoon does 

 enter. On the other hand, the maturation division occurred 

 without a spermatozoon entering, if some potassium was added 

 to the sea-water. Hence the effect of the spermatozoon is in 

 this case not due to the introduction of a centrosome into 

 the egg. 



3. Investigations into the effects of ions had led the writer 

 to believe that in them we possess very potent factors in life 



