296 



SOAfK PKOBLEMS OF CELL-ORGANIZATION 



C, D). Again, the two chromosomes left in the egg after removal of 

 the second polar body may accidentally become separated. In this 

 case each chromosome gives rise to a reticular nucleus of half the 

 usual size, and from each of these a single chromosome is afterward 

 formed (Fig. 143, A, B). Finally, it sometimes happens that the two 

 germ-nuclei completely fuse, while in the reticular state, as is normally 

 the case in sea-urchins and some other animals (p. 188). . From the 

 cleavage-nucleus thus formed arise four chromosomes. 



The same general result is given by the observations of Zur Strassen 

 ('98) on the history of giant embryos in Ascaris. These embryos 

 arise by the fusion, either before or after the fertilization, of previ- 

 ously separate eggs, and have been shown to be capable of develop- 

 ment up to a late stage. Not only in the first but also in some, at 

 least, of the later mitoses, such eggs show an increased number of 



chromosomes proportional to the number 

 of nuclei that have united. Thus in 

 monospermic double eggs (variety bi- 

 valcjis)\\\Q. number is six instead of four ; 

 in dispermic double eggs the number is 

 increased to eight (Fig. 144). 



These remarkable observations show 

 that ivJiatevcr be tJie luiuiber of ehro^no- 

 somes entering into tJie formation of a 

 reticular nucleus, t/ie same number after- 

 ward iss7ies from it — a result which de- 

 monstrates that the number of chromo- 

 somes is not due merely to the chemical 

 composition of the chromatin-substance, 

 but to a morphological organization of 

 the nucleus. A beautiful confirmation 

 %'ar hn'aiem, arising from a double- ^f ^i^jg conclusiou was afterward made 



Jertilized double egg, showing eight , ,-, . , , , 



chromosomes (7.ur strasseji). by Bovcri ( 93, 95, i) and Morgan ( 95, 



4), in the case of echinoderms, by rear- 

 ing larvoe from enucleated egg-fragments, fertilized by a single sper- 

 matozoon (p. 194). All the nuclei of such larvae contain but half the 

 typical number of chromosomes, — i.e. in Echinus nine instead of 

 eighteen, — since all are descended from one germ-nucleus instead 

 of two ! 



Equally striking is the remarkable fact, described at page 275, that 

 all of the cells in the sexual generation (oophore) of the higher 

 cryptogams show half the number of chromosomes characteristic of 

 the sporophyte, the explanation being that while reduction occurs 

 at the time of spore-formation, the spores develop without fertilization, 

 the reduced chromosome-number persisting until fertilization occurs 



Fig. 144. — Giant-embryo of Ascaris, 



