THE HELIOZOA 25 



In the Desmothoraca, of which Clathrulina is the best-known 

 genus, the skeleton has the form of a spherical basketwork, the 

 bars of which often show a median ridge on the outer surface, the 

 spaces between the bars being irregularly polygonal with rounded 

 angles (Fig. 2 (2)). This basketwork is supported on a long, hollow 

 .siliceous stalk. 



The structure of the nucleus and the processes of hiryolcinesis 

 have been minutely described by R. Hertwig (8) in Actinosphaerium, 

 and his descriptions are in accord with what is known concerning 

 them in the higher Heliozoa generally. 



The resting nucleus of Actinosphaerium has a definite membrane 

 .continuous with an internal achromatic network whose relation to 

 the chromatin elements is very variable. The whole of the 

 .chromatin may be collected into a relatively large mass, supported 

 in a matrix of achromatic substance ("plastin") and forming a 

 conspicuous " karyosomatic " nucleolus ; such a condition of the 

 nucleus may be induced by starvation, or it may appear as a pre- 

 liminary to division. In well-fed individuals the chromatin spreads 

 through the nucleus in the form of coarse branches or networks. 



Nuclear division may be direct, in the formation of buds or 

 swarm -spores, or by karyokinesis. Karyokinesis occurs in the 

 division of the nuclei within the body of the multinucleate forms 

 (e.g. Actinosphaerium) without being followed by division of the 

 body ; in forms with a single nucleus it occurs during fission, and 

 during the maturation of the conjugants (gametocytes). 



In Actinosphaerium there are three kinds of karyokinesis, that 

 differ from each other in some details of considerable theoretical 

 importance. In the nuclear divisions of the unencysted body no 

 entrosomes are formed, and the spindle is considerably compressed 

 between the two poles. In both the mitoses of the maturation of the 

 gametocytes, centrosomes occur at each pole of the spindle, but in 

 the first (polar) division the chromosomes are larger than in the second 

 .(polar) division, and there are some other differences in detail of 

 minor importance. In all three kinds of karyokinesis there are 

 numerous chromosomes (about 150), and both the divisions of the 

 nuclei in the maturation of the gametocytes are of the nature of "equa- 

 tion " divisions, the number of the chromosomes not being reduced. 



It may be convenient to describe in greater detail the second 

 polar division of Actinosphaerium as an example of the karyokinesis 

 of the Heliozoan nucleus. At the end of the first polar division, 

 one of the resultant nuclei degenerates and is ultimately ejected 

 with the first polar body, the other remains in the centre of the 

 protoplasm and passes through a short period of rest. At one 

 pole of this resting nucleus there is a clearly-marked centrosome 

 surrounded by a small aster. Antecedent to the second polar 

 division the centrosome diminishes in size (Fig. 5, I, c), and 



