1894. CELL-DIVISION. 425 



This reducing division has excited great attention among biolo- 

 gists, and scarcely a week passes without some new contribution to 

 the already extensive literature on the subject. It is only possible 

 here to give the briefest mention of the view that Weismann now 

 holds. This author considers the nuclear chromatin rods, or 

 " idants," to be composed of individual masses of ancestral plasms, 

 or " ids," each of which could, if it alone dominated the ovum, control 

 the whole course of development, and give rise to a complete indi- 

 vidual of the species. The doubling of the idants before the reducing 

 division is a means of increasing the number of possible combinations 

 among the ids, and of thereby ensuring that as many spermatozoa as 

 possible shall possess the hereditary tendencies balanced in different 

 proportions. By this means, which equally well applies to the ovum, 

 the organism resulting from the union of two individuals may differ 

 markedly from its parents, or, in other words, may exhibit consider- 

 able variation from the specific type. 



Before leaving the subject of the reducing division, it should be 

 noticed that a similar process occurs in the Protozoa. Maupas has 

 shown that the micro-nucleus of the Ciliata, which alone takes any 

 part in conjugation, undergoes as many as three reducing divisions 

 before it is ready for action. In some lowly-organised plants 

 (Desmidiacea) reducing division takes place after two individuals 

 have fused to form a zygote. 



Lastly, Boveri has carefully traced the method ot nuclear- 

 division in the segmenting ovum of Ascaris, and finds that large 

 masses of chromatin may be extruded bodily from the cells. Further, 

 he has discovered that the ovum divides, at its first segmentation, 

 into a somatic-cell and a germ-cell. The difference between them is 

 marked by the way their nuclei divide. Whereas the former cell and 

 its descendants divide in a more or less rough and ready manner 

 (though always karyokinetically), and masses of chromatin are 

 extruded bodily from the cell, the latter divides, with the exactest 

 halving of its rods, into two, one of which halves repeats the division 

 of its parent, while the other segments in a manner similar to the 

 somatic cells. Thus, up to a certain point in ontogeny, there is in 

 the embryo, at any given period, one cell which is a product of perfect 

 karyokinetic division. Beyond this stage all the descendants of this 

 cell divide similarly to it, and give rise to the germ-cells. Thus there is 

 a perfect continuity of the nuclear matter from the two-celled stage in 

 development up to the ripe sexual elements of the adult. Boveri's 

 results are, therefore, a striking proof of Weismann's theory of the 

 continuity of the germ-plasm, as far as outward form goes. 



The above account deals with some of the main points in which 

 cell-, and more particularly nuclear, division are of importance, as 

 regards both the actual process of fertilisation and development. It 

 is naturally impossible to give any clear idea of Weismann's 

 elaborate theories based on nuclear structure and the changes it under- 



