ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 6 ( J 



©f development into an embryo-sac. In the embryo-sac three divisions 

 then succeed one another, the last of which gives riso to the ovum- 

 nucleus. There are here, therefore, four stages in the reduction of the 

 number of chromosomes. (2) An embryo-sao mother-cell divides into 

 two daughter-cells, of which either only the upper or only the lower is 

 capable of developing into an embryo-sac. In the embryo-sac three 

 further divisions take place, the last of which gives birth to the ovum- 

 nucleus. The number of stages in the reduction is only three. (3) An 

 ombryo-sac mother-cell develops directly into an embryo-sac, in which 

 three stages in the process of division are required for the production 

 of the ovum-nucleus. There is therefore a further reduction in the 

 number of stages. 



In the first type (Galtonia candicans, Convallaria majalis), the hetero- 

 type and homotype divisions of the nucleus are completed in the two 

 progamic generations which precede the formation of the embryo-sac ; 

 in the embryo-sac three typical nuclear divisions succeed one another. 

 In the second type (Scilla sibirica, Allium ursinum) only the heterotype 

 nuclear division is progamic ; the homotype division represents the first 

 nuclear division in the embryo-sac ; the two following divisions in the 

 embryo-sac are typical. In the third typo (Tulipa Gesneriana) both 

 heterotype and homotype divisions take place in the embryo-sac ; only 

 the third nuclear division is typical. 



The result is that the reduction of the number of chromosomes in 

 the nucleus of the embryo-sac mother-cell causes in it a heterotype 

 division ; this is succeeded by a homotype division, and only the sub- 

 sequent divisions are typical. These observations point to the conclu- 

 sion that the reduction in the number of chromosomes in the rudiment 

 of the seed is the point of time of the birth of the new generation. 



Double Fertilisation in Naias.* — L. Guignard describes another 

 case of double fertilisation in the case of Naias major. The formation 

 of the egg-apparatus in the embryo-sac takes place in the usual way. 

 The nuclei of the pollen-mother-cells and those derived from them 

 present the phenomenon of chromatic reduction ; instead of containing 

 twelve chromosomes, they possess only six, the smallest number which 

 has as yet been found in the sexual nuclei of plants ; and these present 

 greater differences than usual in length in the same nucleus. Similar 

 characteristics are presented by the nuclei of the embryo-sac. The 

 three antipodals are not alike ; the two lower ones are bounded by cell- 

 walls, and are distinguished from the upper one by their smaller nuclei ; 

 while they become gradually absorbed, this latter, which has no limiting 

 wall, continues to grow, and persists, after impregnation, at the base of 

 the embryo-sac. The two polar nuclei do not fu^e until shortly before 

 impregnation. One of the synergids is also absorbed considerably 

 before the other one. The fusion of the male with the female nuclei 

 takes place with great rapidity, and is very difficult to follow. 



After impregnation, contrary to what usually occurs, the division of 

 the ovum-cell takes place before that of the secondary nucleus. Two 

 embryos were several times observed in an embryo-sac, presenting all 

 the characters of normal embryos formed by impregnation. 



* Journ. de Dot. (Morot), xv. (1001) pp. 205-13 (15 figs.). 



