STRUCTURE OF THE SEGMENTATION NUCLEI. 31 



b. A form closely resembling the above, except in the fact 

 that the nuclear spongework is stained slightly, though not quite 

 so deeply as, some of the extra-nuclear protoplasm (PI. Ill, 

 fig. 2). There are only three (in the whole nucleus) small 

 deeply-staining masses, which are not so conspicuous as in the 

 first form, but are more deeply stained than the membrane and 

 septa. 



Using the second of the two above-mentioned alternatives, 

 we may state the difference between these two nuclei thus : in 

 the first form the chromatin of the nucleus is aggregated into 

 a number of small masses, while in the second form the chro- 

 matin is, for the most part, diflPused throughout the nuclear 

 reticulum. The word chromatin being used to denote the 

 property which enables the protoplasm to take up and retain 

 the staining matter. The extra-nuclear protoplasmic threads 

 possess this property, and may be said to possess chromatin, 

 but it is in a diffused form, as in the second form of nucleus. 



c. In the third form (PI. Ill, fig. 3) the nucleus is divided 

 by a number of septa, radiating from its centre, into chambers. 

 The chambers are partially divided up into secondary chambers 

 by prolongations of the septa. The septa are continuous 

 externally with the extra-nuclear protoplasmic reticulum. It 

 is impossible to speak of a distinct boundary of the nucleus in 

 this form, and the substance of the nuclear septa and of their 

 prolongations is exactly similar in appearance and staining 

 properties to the strands of the surrounding protoplasmic net- 

 work or spongework. 



A number of chromatin masses occur in each chamber of 

 this radiate nucleus — they appear to lie in the offshoots of the 

 septa into the chambers and in delicate expansions of these. 

 But it is impossible to determine exactly the relation of 

 these chromatin globules to the protoplasmic network in the 

 nucleus. 



This form of nucleus is most interesting, because were it not 

 for the chromatin masses the nucleus would be quite undistin- 

 guishable from the surrounding protoplasm, except, perhaps, 

 by the fact that the meshes of the network (i. e. network as 



