100 



BIOLOGY OF THE PROTOZOA 



roll is. In the latter case, also, there is a peculiar lagging hetero- 

 chromosome ("axial chromosome") of unknown significance. 



(6) Origin of Fertilization (Meiotic) Chromosomes. — In practically 

 all Protozoa the sequence of stages leading to formation of chromo- 

 somes which enter into pronuclei is quite different from that of the 

 division nuclei. This phenomenon is one of the final acts of develop- 

 ment and in Protozoa represents a last stage of differentiation of 



A 



Fig. 54. — Triehonympha campanula in division. A, and B, prophase and anaphase 

 of nuclear division; the divided centroblepharoplast forms the poles of the spindle 

 and are connected by a paradesmose. C and D, breaking up of chromosome spireme 

 into chromosomes which show a tendency to unite in pairs. (After Kofoid and 

 Swezy.) 



the derived organization of the nucleus. Here, as in Metazoa, there 

 are at least two maturation divisions, while in ciliates the number 

 is increased to three. As in Metazoa, one or the other of the matura- 

 tion divisions is a reducing division or reduction may be parcelled 

 out in both divisions, the end-result being that the number of 

 chromosomes is reduced by one-half, i. e., from the diploid to the 

 haploid number. As in Metazoa, the first of the meiotic divisions 



