152 COMPARATIVE ANATOMY 



nucleus into two equal halves, each of which contains a pole- 

 plate, a half of the spindle, and a chromatic plate. From 

 each half a daughter-nucleus is reconstituted. This description 

 only applies to the nuclear division of free living Actinosphaeria ; 

 in the processes about to be described three other varieties 

 of mitosis have been observed, the details of which need not 

 occupy our attention except in so far as they are of importance 

 for the understanding of the phenomena under consideration. 

 At the onset of winter Actinosphaerium ceases to multiply 

 by binary fission, and undergoes a process known as en- 

 cystment with spore formation. The axial filaments of its 

 pseudopodia are absorbed; the pseudopodia themselves are 

 withdrawn ; the vacuoles in the cytoplasm disappear ; and a 

 gelatinous, sticky envelope of relatively considerable thickness 

 is secreted as a protective coat to the protoplasm. A number 

 of round or oval platelets, or granules with thickened edges, 

 make their appearance in the cytoplasm, rendering it turbid 

 and opaque. These granules seem to serve as reserve material 

 for the spores which are shortly afterwards formed, and they 

 may therefore be called yolk-granules. At the same time 

 a number of minute siliceous spicules are formed in the 

 cytoplasm, and the majority of the nuclei disintegrate and are 

 absorbed, only about five per cent, of the original number 

 remaining. These remaining nuclei increase in size, and 

 presently the cytoplasm is divided into as many corpuscles 

 as there are nuclei, each corpuscle surrounding itself with a 

 special gelatinous capsule into which the siliceous spicules 

 previously scattered through the cytoplasm are collected. 

 These corpuscles are known as the primary cysts. The 

 nucleus of each divides mitotically, and division of the nuclei 

 is followed by the division of each primary cyst into two 

 secondary cysts which are destined to become conjugating 

 cells or gametes, and may therefore be described as gameto- 

 cytes. The mitosis of the nuclei of the primary cysts differs 

 from that already described in the fact that centrosomes are 

 formed from the chromatin of the nuclei. The secondary 

 cysts remain side by side in the pairs in which they were 

 formed, and the nucleus of each divides twice to form two 

 polar bodies. The process may be shortly described as 

 follows : The nucleus divides mitotically, and one of its 

 halves is expelled from the cyst as the first polar body. The 



