20 Overton . — On the Organization of the Nuclei in the 
telophases and prophases of succeeding divisions, my studies thus confirming 
the recent conclusions of several authors that the chromosomes remain 
separate during these phases. These plants also show the arrangement and 
position of the chromosomes in the pollen grain, which is of interest as 
showing the chromosomes arranged in a single series. I have also studied 
the pollen mother-cells of Richardia africana in all stages of development, 
and shall describe the stages which bear upon the above points. 
The structure of the resting nucleus usually described for plants and 
animals, in which there is a net-work consisting of a fine, rather uniformly 
distributed linin and chromatin reticulum, is quite unlike that found in these 
plants. The limits of the individual chromosomes are not lost to view in 
the resting nucleus. Neither does the structure of these resting nuclei 
correspond to the conception that the reticulum consists of a single sub- 
stance, as urged by Gffigoire and Haecker, who hold that it is arranged into 
a reticulum, which stains more densely at the junctures of the threads, to 
form so-called net-knots. I find thus that the behaviour of the chromosomes 
during the prophases of division of the pollen mother-cells may be followed 
in detail on account of their continuous presence as definitely limited bodies. 
This makes it possible to establish the existence of a pre-synaptic bivalent 
spirem extending back to the resting nucleus, as against the contention of 
Farmer and Moore and Mottier. In most plants and animals studied, the 
limits of each individual chromosome are not to be determined during the 
spirem stage, a fact which makes it difficult to follow the behaviour of each 
individual chromosome. The heterogeneous post-synaptic spirem of the 
plants consisting of chromosomes and intervening linin segments shows its 
double nature from its formation. The limits of each somatic pair of 
chromosomes may be determined, since at no time is there a continuous 
chromatic post-synaptic spirem formed. Several authors, Sargant (’96, ’97), 
Guignard (’99), Gregoire (’99), Strasburger (’00), and Mottier (’03), have 
described a splitting of the daughter chromosomes in the prophases to form 
the tetrads. They describe the appearance of a double series of chromo- 
somes, one in each half of the longitudinally-split spirem or daughter 
chromosomes of diakinesis. 
Calycanthus floridus and Richardia africana show that the tetrads are 
definitely differentiated by the longitudinal splitting of each component of 
the diakinetic pairs of chromosomes, resulting in four elongated rodlets, 
lying parallel, two of which have arisen from one chromosome and two from 
the other. The further history of these tetrads has been followed through- 
out the formation of the spindle and division stages. 
The fixing fluids giving the best results for the pollen mother-cells of 
Thalictrum purpurascens are the alcohol-acetic and the alcohol-acetic-chloro- 
form mixtures of Carnoy, which proved to be good for all stages. I have 
thus been able to obtain good fixation for all post-synaptic stages, which 
