484 BIOLOGY OF THE PROTOZOA 



colonies with definite form and a definite number of cells {(ionium, 

 Eudorina, VJaiydornia, etc.)- Here phenomena are manifested 

 which may well be compared with cleavage and differentiation of 

 the metazoon egg, the resultant cells taking an in\ ariable position 

 in the aggregate. A colony of (ionium jwctornir for example com- 

 posed of 1() similar cells, upon reproduction produces 10 colonies. 

 Each of the original cells undergoes a fairly regular cleavage, the 

 cells adhering until a comparatively late stage and finally forming 

 the flattened plate of cells characteristic of the species. The 

 colony Platydorina is even more remarkable in the axial relations 

 of the 32 cells composing it (Fig. 3 A, p. 21). In Pleodorina 

 {Evdorina) the cells are differentiated into somatic and germinal, 

 and in Volvox the somatic cells form a tissue while the germinal 

 cells are enclosed in the inner jelly, and in the same genus, finally, 

 some colonies form only male gametes and others only female. 



The changes in form and structure with growth are to be traced 

 to changes in the protoplasmic organization M'hich in turn are 

 doubtless due to metabolic activities and there is evidence that 

 analogous changes are resj)onsible for the differentiations which 

 accompany regeneration in the more actively developing ciliates. 

 In this connection the merotomy experiments of Calkins (1911) 

 and Young (1922) are suggestiAe. In Chapter V it is shown that 

 anticipatory changes in the cell precede the nuclear changes. This 

 was first demonstrated by Wallengren(lSOO) for Sti/loni/chia and 

 Euplotes, and is clearl\' shown in Uronychia transfuga in which the 

 new posterior giant cirri are formed sometime prior to the nuclear 

 changes in jjreparation for division. The new cirri appear in a 

 region of the cell previously free from cirri, as well as at the bases 

 of the old cirri. Similarly there is a complete new formation of 

 the peristome with membra nelles in the posterior half and a new 

 series of membranelles which rejDlace the old ones in the anterior 

 region. Except for mutilations these regenerations and replace- 

 ments occur only at periods antecedent to cell division and indicate 

 some far-reaching change in the constitution of the protoplasmic 

 make-up. The ability to undergo such a change furthermore is 

 progressive as shown by experiments in cutting Uronychia (Calkins 

 1911). In these experiments the cell if cut immediately after divi- 

 sion in a plane indicated by the section line (Fig. 202), is divided 

 into two fragments one of which, the posterior with giant cirri, 

 contains the single micronucleus, while the anterior portion, with 

 peristome, contains a part of the macronucleus but no micronucleus. 

 In such cases the anterior portion may live for four or fi^ e days as 

 an amorphous fragment but it ne\'er regenerates the giant cirri. 

 The ])osterior part however, regenerates the missing anterior region 

 within a few hours and becomes a perfect cell. Exactly the same 

 result invariablv follows if an indi\idual is cut when five to eight or 



