302 



BIOLOGY OF THE PROTOZOA 



plate is formed with 48 to 50 chromosomes. With the first division 

 these separate into two groups, each with 24 chromosomes (Fig. 151). 

 3. Phase C. The Second Meiotic Division. — Prior to Prandtl's 

 work on Didinium there were no conclusive observations on the 

 reduction of chromosomes in ciliates. He found that the 1(3 chromo- 

 somes characteristic of the first maturation division become reduced 

 to 8 with the second division. Since his work appeared there has 

 been a number of authentic observations along the same line. Thus 

 Enriques (1907) found a reduction in number from 16 to 8 chromo- 

 somes in Opercularia coarctata and the same observer (1908) de- 

 scribed a reduction from 4 to 2 in Chilodon uncinatus (Fig. 149), 

 reduction occurring at the second division. Other cases of the same 

 type are Carchesium polypinum (Popoff, 1908) with reduction from 

 16 to 8; Anoplophrya branchiarum (( lollin), from 6 to 3; and Uroleptus 



l ! 



c 



Fig. 152. 



■Uroleptus mobilis. The second meiotic division and reduction in number 

 of chromosomes during conjugation. (After Calkins.) 



(Calkins, 1919) from 8 to 4 (Fig. 152). In all cases the second 

 meiotic division appears to be unaccompanied by any of the pre- 

 liminary activities which characterize the first division. In some 

 the nuclei do not return to a resting condition between the two 

 divisions, but in other cases, c. g., Chilodon (MacI)ougall, 1925), 

 the second spindle forms from a resting nucleus. 



In ciliates with a multiple number of micronuclei the number par- 

 ticipating in the second division appears to bear no constant rela- 

 tion to the number derived from the first division. In cases having 

 but one micronucleus in the vegetative stages the numerical rela- 

 tions are fairly constant, two spindles in the second meiotic division 

 being the rule. There are, however, some exceptions. Thus in 

 Paramecium bursaria, according to Hamburger (1904), one of the 

 nuclei formed by the first division degenerates without forming a 

 spindle so that only one nucleus undergoes the second division. 



