256 BERTRAM G. SMITH. 



Hertwig the cleavage-nucleus or segmentation-nucleus, later 

 observations showed that such is not always the case. In 1881 

 Mark demonstrated that in the slug Limix the two germ-nuclei 

 do not fuse; after coming together they persist during the forma- 

 tion of the two cleavage centers, then their membranes gradually 

 disappear. Two years later Van Beneden showed that in Ascaris, 

 not only do the germ-nuclei become apposed without fusion, but 

 each gives rise to an independent group of chromosomes which 

 contribute equally to the formation of the daughter-nuclei. 

 Thus the foundation of the doctrine of the biparental character 

 of the nuclei of sexually-produced organisms was laid down by 

 Van Beneden. In many animals, and in some plants (gymno- 

 sperms), the independence of the formation of the maternal and 

 the paternal chromosome groups following fertilization has been 

 established by direct observation, though the demonstration has 

 seldom been carried beyond the first cleavage stage. On the 

 other hand there are some animals, and many plants, in which 

 the germ-nuclei meet and fuse while in the resting condition, so 

 that in the chromatin of the resulting nucleus maternal and 

 paternal contributions cannot be readily distinguished. 



Hacker (1892 and 1895) an d Riickert (1895) found that the 

 germ-nuclei of Cyclops do not fuse but preserve their individuality 

 throughout at least a considerable period of the cleavage of the 

 egg. In mitosis the two groups of chromosomes, of maternal 

 and paternal origin respectively, remain distinct and bilaterally 

 distributed, while each resultant daughter-nucleus in the resting 

 stage consists of two closely apposed but structurally separate 

 vesicles. 



In 1901 Conklin described the double structure of the cleavage 

 nuclei of Crepidula in certain stages of the nuclear cycle, and 

 pointed out that the two halves which at times appear as distinct 

 entities are almost certainly to be regarded as of maternal and 

 paternal origin respectively. "This separateness is most easily 

 observed in the telophase of each division, though in some cleav- 

 age cells it may be seen in the prophase also, or even throughout 

 the resting period. At the time when the daughter-nuclei are 

 being formed the chromosomal vesicles fuse into two groups 



