CELL-NUCLEUSFERTILISATION 241 



fertilisation recurs meiosis is never omitted, 1 and 

 this is true for animals as well as for plants. 

 The ordinary course of life histories has been 

 developed long after sexuality and meiosis 

 appeared, and has progressed independently, 

 and on different lines in different groups. 

 Sometimes, as in the higher plants, the stages 

 of the life history are more or less obviously 

 connected with these nuclear cardinal points, 

 at other times the relation is not so evident. 

 For example, it may happen, as in many of 

 the flowering plants, that the two stages in 

 the life history so well separated and analysed 

 in the fern, become curtailed. This happens 

 when the cell which should give rise, by the 

 two divisions, to four spores, cuts the process 

 short, grows, and itself becomes the spore 

 without any division. Such a short cut is 

 taken in certain of the sporangia (ovules) of 

 a lily, orchis, and many other plants. But 

 meiosis is not cut out. It supervenes at the 

 very next divisions which follow the omitted 

 stages during which it would normally have 

 been effected. 



As we advance to types of plants above the 

 ferns we find the life history becoming more 

 complicated and less diagrammatically clear. 

 The principle which underlies the complica- 

 tion is, however, a simple one; it consists 

 in a provision for giving the sexually produced 

 embryo an advantageous start in life. 



1 The occasional anomaly reported for certain mosses 

 requires further investigation. 



Q 



