oh' THE GERM CELLS OF METAZOA. 



1C>1 



the chromosomes equatorially. In the anaphase of the first maturation division the con- 

 striction of the chromosomes generally has the appearance shown in Fig. 195 of my 

 former paper; while Fig. 196, which I then considered to represent the typical condition, 

 I now, from the study of more abundant material, find to be an unusual condition. Thai 

 is to say, the appearance of the chromosomes shown in Fig. 196 of my preceding paper 

 is really atypical, since in this case their constrictions appear at right angles to the long 

 axis of the spindle, whereas in most other cases the planes of these constrictions coincide 

 with planes passing through the long axis of the spindle. In this second maturation 

 division the chromatin nucleolus is not always divided. 



2. Euohistus tristigmus Say 



Four testes of this species were studied. 



In the rest stage of the spermatogonium there are two small chromatin nucleoli, 

 generally attached to the surface of the true nucleolus. 



In the spermatogone mitosis there are fourteen chromatin segments in the equatorial- 

 plate (PI. I, Fig. 20); the twelve larger, usually somewhat elongate ones are chromo- 

 somes, and the two smallest, rounded ones are chromatin nucleoli. All these elements 

 arc halved in meta kinesis. 



In the synapsis stage the twelve chromosomes unite to form six hi val en t chromo- 

 somes. The two chromatin nucleoli sometimes unite to form a bivalent one, which is 

 clearly dumhhell-shaped in earlier stages, but in the resting spermatocyte hecomes 

 rounded and has a peripheral position ( Fig. 22) ; or quite as frequently they remain 

 separate from one another during the growth period, and are seen to he of unequal vol- 

 umes (Fig. 21). The chromatin nucleoli in the growth period are rarely attached to the 

 true nucleolus. 



In the first maturation division there are always six clearly bivalent, dumbbell- 

 shaped chromosomes and either one dumbbell-shaped bivalent chromatin nucleolus or, 

 apparently more frequently, two univalent, chromatin nucleoli of more or less rounded 

 form and different volume (lateral view shown in Fig. 25, N. 2). Accordingly, on pole 

 views of the monaster stage there are seen either seven chromatin elements (Fig. 24), 

 which are six bivalent, chromosomes and one bivalent chromatin nucleolus, or there are 

 eight, namely, six bivalent chromosomes and two univalent chromatin nucleoli (Fig. 23: 

 in this figure one of the chromatin nucleoli can he distinguished by its smaller size, but 

 which of the remaining seven elements is the other chromatin nucleolus is not easily dis- 

 cernible on pole views, since the larger of the two chromatin nucleoli has a diameter 

 equal to that of one of the smaller chromosomes). 



All the six chromosomes are halved (by a reduction division) in metakinesis, so that 



