196 THE CELL 



but is seen most distinctly at that period when it has extended 

 itself around one third of the circumference of the egg to be pro- 

 vided with a large number of small grooves, "which open into it on 

 both sides for the most part at right angles (60-100 on either side, 

 Fig. 92). Thus a very pretty picture is produced, like a long deep 

 valley in the mountains, with a large number of shorter, narrower 

 valleys opening into it on either side. As the process of division 

 progresses, and the main furrow deepens, the side furrows diminish 

 in number, and finally quite disappear. 



The appearance of this peculiar and clearly marked coronal fur- 

 row is a phenomenon which is connected with the contraction of 

 the protoplasm during cleavage. 



c. Division of Plant Cells. The protoplasmic coating of 

 the wall of, the embrj'o-sac of Fritillaria imperialis affords an in- 

 structive illustration of the great uniformity of the process of 

 nuclear division as it occurs in plants and animals. This, as well 

 as the embryo-sacs of other Liliacece, is particularly suitable for the 

 study of nuclear figures, for the layer of protoplasm is extremely 

 thin, and, if examined at the right time, is seen to contain a large 

 number of nuclei at various stages of division (Strasburger VI. 

 71-73; Guignard VI. 23). 



The large resting- nucleus contains a linin framework with small 

 meshes (Fig. 93 A), upon the surface of which a large number of 

 small nuclein granules are pretty evenly distributed. In the 

 majority of cases nucleoli are present. These vary in size, and 

 lie between the meshes of the framework, to which they are 

 attached. Strasburger is of opinion that, when the nucleus is 

 preparing to divide, the whole framework becomes transformed 

 into a few fairly thick threads, which are much twisted ; he de- 

 scribes in them a diagonal striation (c) similar to that observed by 

 Balbiani (II. 3) in the nuclei of Chironomus larvae (Fig. 27). He 

 accounts for this striation by the statement, that each thread is 

 composed of numerous discs of nuclein arranged one after the 

 other, and separated by their partition walls of linin. 



In the course of time, as the process advances, the nuclear mem- 

 brane dissolves, and the nucleoli break up into smaller granules and 

 disappear, whilst the nuclein threads grow shorter and thicker, and 

 produce twenty-four nuclear segments ; a typical spindle composed 

 of a large number of most delicate fibrils develops, in the centre of 

 which the nuclear segments arrange themselves in a circle (Fig. 

 93 D). Guignard has lately demonstrated the presence of two 



