2i8 BIOLOGICAL LECTURES. 



of the Flagellata with intranuclear sphere where, as for exam- 

 ple in Eiiglena, the granules become welded together into 

 short, thickened chromosomes, but there are no observations 

 to show that longitudinal division takes place. A somewhat 

 similar process of chromosome formation is described by Hert- 

 wig ('98) for Actinosphceruim. In the latter case, as in Nocti- 

 luca and Etiglena, the number of chromosomes is very great, 

 and it is impossible to state whether the number is constant 

 or not. 



In none of these cases is there a preliminary arrangement of 

 the chromatin into a definite spireme, as in the higher forms. 

 In Noctihica an occasional thread-like arrangement of the 

 granules is observed, but it does not appear to be constant. 

 In numerous other Protozoa, however, spireme formation may 

 become as complex as in Metazoa, and, as in the latter, the 

 thread may be segmented into chromosomes of definite shape 

 and size, although usually of large number {EnglypJia, Opa- 

 Una, Actinophrys sol, AcantJiocystis, Ceratium, etc.). 



Turning now to the so-called achromatic structures of the 

 cell, we find that the origin of the metazoan centrosome and 

 attraction sphere from simpler elements in Protozoa has been 

 the subject of a number of interesting theories. Most of these 

 undertake to work out the morphogenesis of the centrosome as 

 it appears in Metazoa. Biitschli was the first to seek its homo- 

 logue in the micronucleus of Ciliata, a comparison which has 

 found its way in nearly all of the subsequent theories. It was 

 quickly caught up by Hertwig and by Heidenhain, and the 

 latter, especially, elaborated it into a complicated theory of 

 phylogeny, the main thesis being that the central spindle, as 

 described by Hermann, is derived from the micronucleus by 

 loss of chromatin, and becomes secondarily connected with the 

 chromosomes of the other nucleus. Lauterborn improved the 

 theory by assuming that micronucleus and centrosome may 

 have had a common ancestor in some primitive binucleated 

 protozoon, such as Avioeba bimicleata (Schaudinn), and that 

 intermediate stages may be seen in certain existing Protozoa, 

 such as Paramoeba Eilhardi (Schaudinn) and Noctilnca. He 

 considered the micronucleus to be a differentiation of one of 



