156 COMPARATIVE ANATOMY 



wall betray its double origin, but it soon becomes spherical, 

 and forms a single chamber, containing the two closely-apposed 

 zygotes. These are separated by a sharp line of division 

 at the surface of contact, and, whilst the cyst-wall is being 

 formed, each forms within its substance a number of minute 

 highly-refracting granules, which give a blue colour when 

 treated with sulphuric acid and iodine, and are therefore 

 probably composed of a starchy substance akin to cellulose. 

 It is supposed that these granules serve as a reserve of food 

 material during the changes about to be described. When 

 the cyst-wall is established, the nucleus of each zygote travels 

 to the side farthest from the partition wall, and there 

 elongates to form a mitotic spindle. The chromatin is 

 gathered at the equator of the spindles as a number of very 

 minute chromosomes, mitotic division takes place, and one 

 half of the divided nucleus is extruded from the cell as a 

 polar body. The other half, reduced to nearly half the size 

 of the original nucleus, travels back towards the partition 

 separating the two zygotes, and lies close against it. The 

 middle part of the partition is absorbed, and the nuclei of 

 the two zygotes meet in the hole thus formed, and fuse 

 together. The next stage has not been seen, but it seems 

 certain that the combination nucleus, formed by the union 

 of the nuclei of the two zygotes, divides, and each product 

 of division passes back to the centre of the body of a 

 zygote. Here it divides repeatedly by mitosis, and thus 

 a number of nuclei are produced, peripherally placed, 

 and each surrounded by an aggregation of denser proto- 

 plasm. Eventually the nuclei, with the denser protoplasm 

 around them, become separated off as corpuscles, or sporo- 

 blasts, and the remainder of the protoplasm of each 

 zygote disintegrates and disappears. Probably it goes to 

 form reserve material. Each of the sporoblasts now sur- 

 rounds itself with a lemon-shaped coat,* and becomes a 

 spore of the kind commonly known as a pseudonavicella 

 (fig. 32), from its supposed resemblance to a boat or canoe. 

 The nucleus of each spore now undergoes three successive 

 nuclear divisions, giving rise to eight minute curved bodies 

 arranged round a central core of residual protoplasm, much 



* The chemical composition of this coat is not known. It is not 

 siliceous as was once supposed, nor does it appear to be chitinous. 



