4 THE DURATION OF LIFE. 



eggs and die immediately after leaving the cocoon ; so that while 

 the former live for many days, the latter do not last for more than 

 twenty-four hours. ' The parthenogenetic form of Solenobia tri- 

 quetrella, soon after emergence, lays all her eggs together in the 

 empty case, becomes much shrunken, and dies in a few hours.' 

 (Letter from Dr. Speyer, Rhoden.) 



NOTE 10. INTERMINGLING DURING THE FISSION OP UNICELLULAR 



ORGANISMS *. 



Fission is quite symmetrical in Amoebae, so that it is impossible 

 to recognise mother and daughter in the two resulting organisms. 

 But in Euglypha and allied forms the existence of a shell 

 introduces a distinguishing mark by which it is possible to 

 discriminate between the products of fission ; so that the offspring 

 can be differentiated from the parent. The parent organism, 

 before .division, builds the parts of the shell for the daughter form. 

 These parts are arranged on the surface of that part of the proto- 

 plasm, external to the old shell, which will be subsequently separated 

 as the daughter-cell. On this part the spicules are arranged and 

 unite to form the new shell. The division of the nucleus takes 

 place after that of the protoplasm, so that the daughter-cell is for 

 some time without a nucleus. Although we can in this species 

 recognise the daughter-cell for some time after separation from 

 the parent by the greater transparency of its younger shell, it is 

 nevertheless impossible to admit that the characteristics of the two 

 animals are in any way different, for just before the separation of 

 the two individuals a circulation of the protoplasm through both 

 shells takes place after the manner described in the text, and 

 there is therefore a complete intermingling of the substance of 

 the two bodies. 



The difference between the products is even greater after trans- 

 verse fission of the Infusoria, for a new anus must be formed at 

 the anterior part and a new mouth posteriorly. It is not known 

 whether any circulation of the protoplasm takes place, as in /.>- 

 glypJia. But even if this does not occur, there is no reason for 



1 Cf. August Gruber, 'Der Theilungsvorgang bei Euglypha alveolata,' and 'Die 

 Theilung der monothalamen Bhizopoden,' Z. f. W. Z., Bd. XXXV. and XXXVI., 

 p. 104, 1881. 



