142 THE THEORY OF THE GENE 



as normal embryos. Boveri and others have studied ex- 

 tensively fragments of sea urchins' eggs, most of which 

 were probably smaller than half an egg. Boveri concluded 

 that these haplonts die, for the most part, before the 

 gastrulation stages or soon thereafter. It is possible that 

 these "fragments" never entirelv recover from the 

 operation, or that they do not contain all the essential 

 constituents of the cytoplasm. 



A comparison of these embryos with those obtained by 

 isolating blastomeres of normal diploid eggs has certain 

 points of interest. It is possible by means of calcium-free 

 sea water to isolate the first two, or the first four, or the 

 first eight blastomeres of the segmenting egg of the sea 

 urchin. Here there is no operative injury, and each cell 

 has the double number of chromosomes. Nevertheless, 

 many of the y 2 blastomeres develop abnormally, fewer 

 still of the 14 blastomeres produce plutei, and probably 

 none of the y s blastomeres pass beyond the gastrula 

 stage. This evidence shows that, aside from the number 

 of chromosomes and from the nucleo-plasma ratio, small 

 size in itself has a deleterious influence. What this may 

 mean is not known, but the surface relations to the vol- 

 ume vary with the size and may possibly enter into the 

 result. 



These experiments do not hold out much promise of 

 obtaining normal vigorous haplonts by diminishing arti- 

 ficially the amount of the protoplasm of the eggs in spe- 

 cies already adjusted to the diploid condition. Neverthe- 

 less, under natural conditions there are several cases 

 known where haplonts exist, and there is one case re- 

 corded where a haplont of a diploid species has reached 

 maturity. 



Blakeslee discovered a plant, in his cultures of Datura, 

 that was haploid, Fig. 84. With care it was kept alive and 

 by grafting upon diploid plants it has been maintained 



