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POPULAR SCIENCE MONTHLY. 



of Loeb that the eggs of the sea-urchin 

 may be caused to develop, without the 

 influence of the male element, by treat- 

 ment with solutions of magnesium chlo- 

 ride or other substances added to the 

 sea-water. Wilson has now examined 

 the internal processes occurring in these 

 esss. Phenomena of this character had 

 been earlier studied by Richard Hertwig 

 and Morgan, whose work paved the way 

 for that of Loeb; but neither of these 

 observers succeeded in obtaining com- 

 plete embryos, the eggs only having 

 passed through the initial stages of de- 

 velopment. Wilson's observations bring 

 the decisive proof that the eggs, devel- 

 oped under these conditions, have not 

 been accidentally fertilized. It is well 

 known that in the fertilization of the 

 egg an equal number of chromosomes 

 are contributed by the egg and the sper- 

 matozoon, this number being in every 

 known case one-half that characteristic 

 of the tissue cells of the species. If, 

 therefore, the magnesium eggs really de- 

 velop without union with a spermato- 

 zoon, we should expect to find them 

 showing but one-half the number of 

 chromosomes occurring in fertilized 

 eggs. Such is, in fact, the case in the 

 magnesium eggs (of ToxopneustesJ, the 

 number of chromosomes being here 18, 

 while in normal fertilization it is 18 

 plus 18, or 36. Every doubt is thus 

 removed regarding the accuracy of 

 Loeb's general result. Interesting light 

 is thrown by the observations on many 

 features of the process of normal fer- 

 tilization. According to Boveri's well- 

 known theory, the egg is induced to de- 

 velop through the importation of a cen- 

 trosome carried by the spermatozoon. 

 In the magnesium eggs this is obviously 

 out of the question; and Wilson's 

 studios, supplementing the earlier ones 

 of Hertwig and Morgan along the same 

 lines, give strong evidence not only that 

 the importation of a centrosome is not 

 necessary to development, but also that 

 the centrosomes of the dividing magne- 

 sium eggs are formed de novo out of the 

 egg-substance. As observed by Morgan, 

 these eggs often become filled with large 



numbers of asters, each of which con- 

 tain a centrosome. One of the most 

 interesting results of Wilson's work is 

 the discovery that these asters may 

 multiply by division and form centers 

 of cytoplasmic division, even when they 

 have no connection with nuclear ma- 

 terial. The important point was deter- 

 mined also that similar asters and cen- 

 trosomes, likewise capable of division, 

 are formed in non-nucleated egg-frag- 

 ments obtained by shaking the eggs to 

 pieces — a fact which shows that the 

 formation of a centrosome may be whol- 

 ly independent of the nucleus. 



In a second paper Wilson described 

 experiments on etherizing normally fer- 

 tilized eggs at various stages, the re- 

 sults of which bear nearly on some of 

 the questions suggested by the mag- 

 nesium eggs. The principal result of 

 these experiments was to show that 

 division of the nucleus and that of the 

 cell-body, though parallel, are in con- 

 siderable measure independent processes, 

 which is in accordance with earlier 

 studies by Hertwig, Demoor and others. 

 The results give, further, considerable 

 ground for the conclusion that the rays 

 of the radiating systems or asters in 

 dividing cells cannot be regarded as 

 fixed, fibrillar structures, as is assumed 

 by most of the prevailing views, but 

 are tracts of protoplasmic flow, as was 

 many years ago maintained by Fol and 

 Butschli. It was shown also that by 

 suitable etherization of the eggs and 

 subsequent transfer to sea-water, the 

 type of fertilization characteristic of the 

 sea-urchin may be artificially changed 

 into that normally occurring in the 

 starfish, and in many worms and mol- 

 lusks; and, in like manner, that the 

 cleavage of the egg may be transformed 

 into a mode that is typical of many of 

 the ccelenterates and arthropods. These 

 observations show that many new and 

 interesting conclusions bearing on the 

 early stages of development may be 

 looked for by further experimental 

 studies along the lines marked out four- 

 teen years ago by 0. and R. Hertwig, 



