218 'I'flOMAS H. BRYOK. 



l)y the prophase figures on the spindle, but in every case a 

 second longitudinal cleavage takes place. 



An examination of his plates shows how remarkably closely 

 the figures I have di'awn for Echinus resemble, even in their 

 details, those he has figured for the ])lant forms, ;ind the 

 general statement of his conclusion is as remai'kably in con- 

 formity with my own. He says (p. 81), "The special peculi- 

 arity of the first nuclear division of the spore and pollen 

 mother cells, which follows the numerical reduction of the 

 chromosomes, consists in this, that the daughter chromo- 

 somes, which ai'ise by a longitudinal splitting of tlie mother 

 chromosomes, are inclined to a premature separation, and 

 that they directly suffer a second longitudinal cleavage. 



"The second nuclear division, which follows on the reduc- 

 tion of the chromosomes, has only the mission of distributing 

 to the granddaughter cells the granddaughter chromosomes 

 already produced in the first division. 



" The two divisions differ from ordinary mitotic division 

 only in the double longitudinal splitting in the first mitosis, 

 and the condition thus created for the second division." 

 Again (p. 99), " '^I'he pith of the heterotypical division lies in 

 the two longitudinal clefts, not in the form of the chromo- 

 somes.-" 



"The cause of the two cleavages of the chromosomes so 

 rapidly following one another, which again conditions the 

 rapid sequence of the two nuclear divisions, must lie in the 

 process of reduction which precedes the maturation division." 



lil'rKi;A'l'MRK. 



AoASSlz, ISOt.— ' li. A^assiz' Coiit.ril). to Nat. Hist. U.S.A.,' vol. v, Caiu- 



biiilf^e, Mass. 

 Balfour, F. M., 1S7S.— 'Quart. Joiirii. ^Micr. Sci.,' vol. xviii. 

 Balfour, F. M., ISSO. — ' Conii)aialivo Emhr^oiogN',' vol.!. 

 VAN Bkneden, J. P., 1841.— 'Ann. (Ic Sc. Nat. Scr.,' tonic ii, 15,p. 120. 

 VAN Beneoen, Ed., 1875.— 'Bull. Ac. Roy. de Iklgiquo,' xi. 

 VAN Beneden, Ed., 1883. — ' Arcli. Biol.,' tome iv. 



