22 C. ISHIKAWA. 



the number present in the maternal nucleus after the expulsion of 

 the second polar body, and in the ripe spermatozoon. 



This number goes unchanged to tlie formation of the first 

 segmentation spindle, which gives rise to two first segmentation 

 spheres, each of which contains eight single chromosomes. 



C. THEORETICAL COXSIDERATIOKS. 



Since the important publications of Anrrhach/^ Ed. van BcnciJcn, 

 Bilt.^clili. the Hrrhvig.'^, Fol, etc., investigations on the phenomena of 

 fertilization ha\e bec<yme more and more interesting, and now after 

 the well known theoretical treatment of tliis and other kindred 

 subjects l)v Trof ]Vei>!)iiann,^-"^ the formation of genital cells, tlieir 

 phases of ripening, find their copulation require exact and thorough 



study. 



The most complete work iri this direction is the beautiful 

 investigation of 0. HerHoig on the " \'ergleich der Ei und Samen- 

 bildung bei Xematoden," published al)out a year ago. In this work 

 is o-iven for the first time a clear insight iiito the exact iinralielism 

 existing between the egg and the sperm-cells, the mode of their 

 development, etc., e\'ery point in which corresponds so exactly with 

 the descriptions given in this paper, that it seems almost superflii("»us 

 for me to have published them. I have deemed it, however, worth 

 while to record the results of my own investigations because the con- 

 clusions arrived a.t 1jy Hertwig are regarded by some authors as 

 demanding explanations other than those he has found for them. 



There are, so far as my knowledge allows me to judge, three 

 modes by which the " Reduktionstheilung" of sexual cells takes place. 

 Platiier'-^^'^^-'' in liis vari<:ius important publications on the sexual cells 

 mentions an exceptional ukmIc of cell-division in the last diNision of 



