POLYGASTEIA. 27 



fine granular appearance, and is much firmer than the loose tissue in 

 which it is embedded. Its colour is commonly a dull yellow by 

 transmitted light ; and in the more minute species it is highly re- 

 fractive : upon the whole it presents the nearest resemblance to the 

 matter of the spermatozoa, which are themselves modifications of a 

 cell-nucleus; and from the close analogy which the so-called "testis" 

 presents to the " nucleus " in both the animal and vegetable cell, it 

 has now generally received the latter name. It is commonly single, 

 and presents the spherical or oval form in Chlatnydomonas {fig' 17. 

 n «.), Eti^lena, Actinophrys, Arcella, Amceba, Bursaria, Paramce- 

 cium, Glaucoma, ^assula, Chilodon, Loxodes {fig- 18. «.), &c. There 

 are usually two nuclei, one behind the other, in Amphileptus Anser, 

 Trachelius Meleagris, and Oxytricha Pellionella : four nuclei have 

 been observed in Stylonychia mytilus. More numerous nuclei, partly 

 united, like a row of beads, have been seen in Stentor polymorphus, 

 Spirostomum ambiguum, and Trachelius moniliger. The nucleus 

 assumes the elongated form in Vorticella convallaria, Epistylis leucoa, 

 and Bursaria truncatella, in which it is slightly bent : in Euplotes 

 Patella it is horse-shoe shaped, and in Stentor Roeselii is spirally 

 twisted.* 



The nucleus performs an essential part in, and seems to govern, the 

 act of propagation by spontaneous fission, which is the most common 

 mode in the Polygastria. In Tab. xxxvi. of Ehrenberg's great work f , 

 fig. vii. 13. shows, in Chilodon cucullulits, the nucleus divided in the 

 direction of the long axis of the body : in fig. 14. the division of the 

 whole body in the same direction has commenced: in fig. 15. it is 

 nearly completed. Figure 16. shows the preliminary division of the 

 nucleus transversely, and the succeeding figures illustrate the trans- 

 verse fission which follows. In most well-fed Polygastria, after the 

 preliminary division of the nucleus, which may be obscured from 

 view by the coloured contents of the body, the first sign of the 

 approaching fission is usually a clear line which may be discerned 

 stretching itself transversely across the middle of the body and indi- 

 cating a separation of the contents into two distinct parts. The 

 containing integument next begins to contract along this line, and 

 the creature to assume the form of an hour-glass (fig. 6, 7. and^g. 14.) : 

 this, though an uncontrollable, seems to be a spontaneous action, and 

 the struggle of each division to separate itself from its fellow indicates 

 an impulse in each to assume its individual and independent character; 

 the which they no sooner effect than they dart off in opposite direc- 

 tions, and rapidly acquire the normal size and figure. In the Vorti- 



* XXrV. p. 24. f XI. 



