46 SEGMENTATION AND FOEMATION OP THE LAYERS. 



matter is a part of the nuclear network, which is itself con- 

 tinuous with the extra-nuclear network. I should be inclined 

 to look upon the process as an increase in size or extension of 

 the nucleus, such as seems to have been described by Strieker 

 in certain leucocytes. 



Pfitzner (14), on the other hand, strongly maintains the isola- 

 tion of the nucleus during the whole of its life-history, and he 

 recommends certain reagents to demonstrate this fact. But 

 inasmuch as he himself admits (p. 72) that these reagents 

 produce great changes in the nucleus, his negative conclusions 

 cannot be regarded as having so good a basis as the positive 

 results of Klein and Leydig, whom I can thoroughly confirm 

 in the matter. 



I may draw attention in passing to the similarity of the 

 branched endodermal nuclei of Peripatus to the nuclei of 

 leucocytes figured by Pfitzner (14, PI. V, fig. 21). 



I have not been able to distinguish nucleoli in the nuclei of 

 Peripatus as distinct from the chromatic thickenings of the 

 spongework. Fleraming (1) says that nucleoli proper partici- 

 pate in forming the chromatic figures in cell division. Flem- 

 ming in his work on the cell and cell nucleus (1) has not seen 

 the continuity between the strings of the nuclear and intra- 

 nuclear spongework. He does not deny its existence but holds 

 that it is not proved. 



Flemming makes the important statement that the first 

 change observable in a cell whose nucleus is about to divide is 

 in the extra-nuclear protoplasm, the fibres of which arrange 

 themselves radially around two points on opposite sides and at 

 the circumference of the nucleus. Contemporaneously with 

 this the nuclear network begins to change, and almost imme- 

 diately afterwards the achromatic spindle-fibres appear in the 

 nucleus. 



These facts seem to point to the conclusion that the actual 

 centre of force, of which the nucleus is the seat, divides first 

 and is followed by the re-arrangement of the cell and nuclear 

 protoplasm. Flemming considers that the nuclear network 

 consists of an achromatic substance containing granules of 



