THE VITAL PROPERTIES OF THE CELL 



263 



Thus in the egg of the round worm of the horse the union of 

 the two sexual nuclei, which is the final stage of fertilisation, 

 only occurs during the formation of the first division spindle, in 

 which process they take an important part. The important 

 principle enunciated by van Beneden is as follows : Half of the 

 nuclear segments of the first division are derived from the egg- 

 nucleus, and half from the sperm-nucleus, hence they may be 

 distinguished as male and female. Now since in this case, as 

 before in nuclear division, the four segments split longitudinally, 

 and then separate, and move towards the two centrosomes, two 

 groups of four daughter-loops are formed, of which two are of 

 male and two of female origin. Each group then transforms 

 itself into the resting nucleus of the daughter-cell. Thus it is 

 indisputably proved, that each daughter-nucleus in each half of 

 the egg produced by the first division process contains two equal 

 quantities of nuclein, one of which is derived from the egg-nucleus, 

 and the other from the sperm-nucleus. 



2. The Fertilisation of Phanero- 

 gamia. The discoveries which have been 

 made concerning the processes of fertilisa- 

 tion in Phanerogamia correspond most 

 completely with those which have been ob- 

 served in the animal kingdom. Stras- 

 burger (VII. 38) and Guignard (VII. 15) 

 stand in the first rank of investigators. 

 The most suitable objects for examination 

 are the Liliaceae, especially Lilium martagon 

 and Fritillaria imperialis. One of the cells, 

 into which the pollen grain divides in 

 Phanerogams, corresponds to the sperma- 

 tozoon, whilst the vegetable egg-cell, which 

 in the ovule is enclosed in the ovary of the 

 gynoscium, forms the most important por- 

 tion of the embryo-sac, and corresponds to 

 the animal egg. 



When the pollen grain has reached the 

 stigma of the style, its contents commence 

 to emerge through a weakened poi'tion of 

 the membrane, and to develop into a long 

 tube (Fig. 143), which penetrates into the 

 style until it reaches an embryo-sac. Here 



FIG 143. Section through 

 the embryo-sac of Lilium 

 mavtagon (after Guignard 

 XV., Pig. 75). At the end of 

 the pollen-tube, whose weak- 

 ened wall is allowing its 

 contents to eecape, the 

 eperm-nucleus may be seen 

 with its two centrosomts. 

 The egg-nucleus is also pro- 

 vided with two centrosomes. 

 On the right, at the end of 

 the pollen tube, a synergida 

 may be distinguished which 

 has commenced to disinte- 

 grate. 



