REPRODUCTION. 641 



of the formation of a polar body. A comparatively simple 

 case is afforded by the Peronosporeae. It was mentioned in 

 the last lecture (p. 619) that the oosphere of these plants is 

 developed from a portion only of the protoplasmic contents 

 of the oogonium, and there is reason to believe that a certain 

 portion of the nuclear substance of the oogonium is excluded 

 from the process. It appears, namely, from Schmitz's re- 

 searches, that the protoplasm of the oogonium is multinucleate, 

 and that during its development nuclear division frequently 

 takes place. The periplasm of the oogonium is certainly 

 nucleated, and it may therefore be regarded as a polar body. 

 In certain of the Algae (Vaucheria, Oedogonium) an extru- 

 sion of a portion of the protoplasmic contents of the oogo- 

 nium has long been known to occur, and something of the 

 same kind has recently been observed by Dodel-Port in 

 Cystoseira barbata. It has not been definitely ascertained 

 whether or not the extruded protoplasmic masses are nu- 

 cleated, but they probably are, and may therefore be con- 

 sidered to be polar bodies. There is no such doubt as to 

 the corresponding cells formed in the female organ of the 

 Muscinese, the Pteridophyta, and of most Gymnosperms. 

 In these plants the central-cell of the archegonium divides 

 into two, a large and a small cell : the former becomes the 

 oosphere, the latter is termed the ventral canal-cell, and sub- 

 sequently undergoes degeneration. There can be no doubt 

 that the latter is a polar body. 



We have yet the case of the Angiosperms to consider. 

 A number of nuclear divisions take place in connexion with 

 the development of the oosphere, but it is not clear which of 

 these is to be taken as indicating the formation of a polar 

 body. The facts are briefly as follows. The nucleus of the 

 young embryo-sac divides into two, one of which travels to 

 each end of the sac : each nucleus then divides, and each of 

 the new nuclei divides again, so that there is a group of four 

 nuclei at each end of the embryo-sac. Of those at the micro- 

 pylar end, one becomes the nucleus of the oosphere, two the 

 nuclei of the synergidae (see fig. 70, p. 616), and the fourth 

 (polar nucleus), which is the sister-nucleus of that of the 



V. 41 



