30 ZOOLOGY FOR MEDICAL STUDENTS chap. 



being followed by cell-division. One of the two daughter nuclei in each 

 case is simply pushed outside the protoplasm and got rid of. The nucleus 

 which remains behind again undergoes mitosis and again one of the two 

 daughter nuclei is extruded. In the place of each secondary cystospore 

 there is now therefore a uninucleate cell with two extruded nuclei 

 lying outside it. These extruded nuclei simply degenerate and play no 

 further part. The two uninucleate cells on the other hand are soon seen 

 to be gametes, for they undergo syngamy with one another to form a 

 zygote. In the place formerl}' occupied by a pair of secondary cysto- 

 spores and at an earlier period by a primary cystospore there is now 

 therefore a zygote. This may be distinguished from a primary cystospore 

 by its smaller size and denser protoplasm and by the cyst round it being 

 thicker and reinforced by minute particles of silica extruded from its 

 cytoplasm. After a prolonged rest, lasting normally for several weeks 

 or months, the zygote — conditions being again favourable — makes its 

 way out and resumes the form of a typical small Actinosphaerium. 



This sexual reproduction of Actinosphaerium is of special interest 

 and importance in two respects, (i) The extrusion of nuclei seen here 

 as an essential integral part of the preparation or maturation of the 

 gametes exemplifies a phenomenon very widely distributed throughout 

 the whole Animal Kingdom. (2) On the other hand a glaring exception 

 is seen here to a very general rule in sexual reproduction namely that 

 the two gametes which conjugate together must not be close " blood- 

 relations." We have here a striking example of extreme " inbreeding.'' 



The general features of the Heliozoa are well exemplified by Actino- 

 sphaerium : the, with rare exceptions, freshwater habitat and the stiff radiat- 

 ing pseudopodia supported by axial filaments are particularly characteristic. 

 The power of secreting particles of silica seen in Actinosphaerium during 

 the reproductive period is much more highly developed in other members 

 of the group, some of which form a definite supporting skeleton of silica. 



Apart from Actinosphaerium the commonest Heliozoan in our fresh- 

 water pools is Actinophrys which is distinguished by its smaller size, its 

 single centrally-placed nucleus and by the extension of the axial filaments 

 inwards right up to the nucleus. 



The group Radiolaria will be dealt with only briefly, not because 

 they are not of great interest but because on the one hand specimens 

 are not readily available for direct study and on the other hand they 

 are not intimately linked up with medical or other studies of immediate 

 practical importance. 



