ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 299 



Formation of Tetrads.* — Starting from the assumption that the 

 embryo-sac of Phanerogams is homologous to tho megaspore of Pteri- 

 dophyta, H. 0. Juel concludes that it must be the product of a tetrad- 

 division, similar to that which gives rise to the spores. This view was 

 confirmed by a series of observations on the development of tho embryo- 

 sac mother-cell in Larix sibirica. This cell divides, by two successive 

 divisions, into four daughter-cells, of which the largest and basal is the 

 embryo-sac. In the first of these divisions the number of chromosomes 

 is only half as large as those in the nuclei of the nucellus, and the 

 same reduced number is found also in the second division. The first 

 division of the nucleus in the embryo-sac mother-cell is heterotypic, 

 corresponding to the first division of the nucleus in the pollen-mother- 

 cell ; the second division is homootypic. The first division of the nucleus 

 in the embryo-sac is typical, agreeing with the divisions of the vegeta- 

 tive nuclei except in the number of chromosomes. It is also distinguished 

 from the two previous divisions in its not being accompanied by cell- 

 division. The conclusion drawn is that the mother-cell of the embryo- 

 sac of Larix is homologous to the mother-cell of a spore or pollen-grain, 

 and that the two cell-divisions, by which the embryo-sac and its three 

 sister-cells are produced, are actual tetrad-divisions. 



The formation of the pollen was followed out in hybrids, especially 

 in one between Syringa vulgaris and S. persica ; and it was found that 

 the almost perfect sterility in the case of this hybrid was due to a 

 great variety of abnormalities in the tetrad-formation — abortion of 

 the pollen-mother-cells before the period of tetrad-division ; irregu- 

 larities in the tetrad-division itself; or an abnormal number of tetrads 

 or of nuclei in the tetrad-cells. 



The Asclepiadeae and Cyperacese differ from the remaining natural 

 orders in the pollen-mother-cells producing each only one pollen-grain. 

 The process was studied in the case of Carex acuta, and it was found 

 that the pollen-grain of the Cyperacese is a very peculiar structure. 

 The membrane in which it is enclosed is that of the pollen-mother-cell, 

 which has become thickened ; the pollen-grain enclosing from an early 

 period an entire tetrad. But in the tetrad it is only a single daughter- 

 cell which developes into a pollen-grain, and this development is in itself 

 perfectly normal. The essential difference from other Angiosperms lies 

 in the fact that in each tetrad the remaining three daughter-cells under- 

 go no further development. 



Development of the Archegone and Impregnation in Tsuga cana- 

 densis.f — W. A. Murrill records the following observations on the 

 hemlock-spruce. The archegones originate as single superficial cells, in 

 each of which occurs the usual division, cutting off an outer smaller 

 cell that forms the neck ; at maturity tho neck usually consists of two 

 cells, though the number varies. In the division of the central cell the 

 spindle fibres arise from a large dense fibrous mass beneath the nucleus, 

 and grow into the nuclear cavity, whore they are joined by fibres from 

 the very small upper pole. As the oosphere nucleus increases in size, 

 and moves to the centre of the oosphere (egg), the vacuole travels 

 -upward towards the apex. Just beneath the ventral canal-cell there 



* Pringsheim's Jahrb. f. wiss. Bot., xxxv. (1900)>p. 626-59 (2 pla.).! 

 t Ann. of Bot., xiv. (1900) pp. 583-607 (2 plB.). 



