THE HELIOZOA 31 



are absorbed, and the animal sinks to the bottom of the water, 

 where it exhibits considerable amoeboid movement, sometimes 

 giving out slender pointed pseudopodia which have no axial 

 filaments ; food-particles are ejected, and a thick, transparent cyst 

 is formed. This " mother-cyst " is of gelatinous consistence, sticky 

 on the outside, and its substance is deposited in concentric layers. 

 The peripheral vacuoles disappear after encystment, and numerous 

 peculiar oval discs, probably consisting of reserve food- material, 

 appear; these bodies may be called "yolk-plates" (Hertwig). 

 While the yolk -plates are forming, the number of the nuclei 

 diminishes, until not more than one -twentieth of the original 

 number remain. The process by which this reduction is effected 

 is not quite clear; Schneider and more recently Brauer (2) 

 have described a fusion of nuclei during the reduction; and 

 Brauer's figures of this fusion are very convincing ; Hertwig, 

 although he considers it not improbable that such a fusion occurs, 

 has never been able to demonstrate it. When the reduction in the 

 number of nuclei is completed, the body divides into as many 

 pieces as there are nuclei, each piece containing a single nucleus. 

 Every result of this division is enclosed in a siliceous " primary 

 cyst," largely formed by rearrangement of scattered spicules 

 secreted before division. The number of primary cysts varies 

 from one to thirty-five ; and Smith (20) has recently shown 

 that there is an interesting relation between the number formed 

 and the temperature at which encystment occurs ; at high tempera- 

 tures the number is smaller and the cysts are larger ; at low 

 temperatures the number of cysts is greater and their diameter 

 less. Smith also shows that the quantity of chromatin contained 

 in the nuclei of primary cysts formed at a low temperature is 

 greater than that found in cysts formed at higher temperatures. 

 Shortly after its formation, each primary cyst divides into two; 

 the nucleus behaves in essentially the same way as dividing nuclei in 

 the unencysted form (cf. p. 25) ; the number of chromosomes is very 

 large, and is estimated by Hertwig at from 130 to 150. The 

 secondary cysts, formed by the division of each primary cyst, now 

 behave like gametocytes ; a centrosome is extruded from the 

 nucleus, and a nuclear division occurs, leading to the extrusion of a 

 first polar body. After the extrusion of the first polar body, the 

 nucleus enters into a resting stage, a single centrosome remaining 

 outside it ; a second division now occurs, leading to the formation 

 of a second polar body, which is in turn extruded. The chief 

 points of interest in the formation of the polar bodies are (1) the 

 similarity of the process of formation, so that neither division can 

 'be called a "reducing division"; and (2) the very pronounced 

 resting stage which intervenes between them. 



After the extrusion of the polar bodies, the two gametes, 



