10 
W. E. AGAR. 
be specially noticed that at present there is no parallelism of 
threads beyond what is certainly accountable for by pure 
accident. 
The next advance is that at one spot of the nucleus (the 
“pole”) these threads become arranged in a parallel manner 
for a short part of their course, and then they immediately 
begin to fuse together in pairs. This is, of course, the 
beginning of the zygotene stage, the interpretation of which 
has been the matter of so much controversy. As is well 
known, one set of workers maintains that each member of 
the pairs of fusing threads is a complete chromosome, and 
that the stage is the one in which numerical reduction of the 
chromosomes takes place (syndesis), while others hold that 
each thread is only equivalent to the daughter halves of a 
precociously and temporarily split chromosome. In describing 
the subsequent maturation processes it is necessary, in order 
to avoid great circumlocution, to use terms which presume 
the truth of one or other of these two views, and as I believe 
that in Lepidosiren the evidence is on the whdle in favour of 
the first view, I have used expressions in accordance with it, 
such as conjugants for fusing leptotene threads, etc. 
The earliest of the three zygotene stages figured is PJ. 1, 
fig. 10. The pairs of leptotene threads have in no case 
fused over more than a small portion of their length. The 
thick threads formed by the fusion of two leptotene filaments 
are, of course, the beginnings of the pachytene threads. In 
some cases the fusing threads can be seen to be twisted 
round one another in a complicated way. Towards the other 
side of the nucleus the conjugating threads diverge, and 
can be traced into the general leptotene complex, but the 
individual threads cannot be traced far owing to their 
complicated courses, and to a certain stickiness which appears 
to make them adhere together where they cross one another. 
As usual the threads have their free ends orientated in the 
same direction, i . e . towards the polar end of the nucleus, 
and it is this pole that fusion is beginning at, and spreading 
away from. Prom analogy with other forms we may probably 
