111 
giving, however, the same results, viz.: the formation of tetrads 
by a primary longitudinal and a secondary transverse division of 
the spireme. The first method (Cyclops, Canthocamptus) agrees 
almost exactly with what I have here described as the “ rod type.” 
The spireme is divided longitudinally before segmentation into 
half the normal number of chromosomes. The chromatin then 
begins to collect into a much thicker double rod; the rod then 
divides transversely and tbe tetrad is formed by longitudinal and 
transverse division. The other method described by Riickert 
takes place in Heterocope and Diaptomus and agrees very closely 
with what I have described as the “ring type.” The double 
spireme breaks as before, but the double segment, instead of re- 
Maining contiguous throughout, becomes separated in the middle, 
while the ends alone remain in contact. A ring is thus formed 
and tetrads arise later by two divisions, one through the diameter 
of original cleavage, the other at right angles to this line. 
Enough has been given in this account to show that certain 
methods tetrad-formation are characteristic. of certain species of 
animals. Up to the present time it has always been found that 
the tetrads in a single nucleus are formed by one method, either 
by the “rod type” or by the “ring type” alone. My observa- 
tions on the fern, however, show that tetrads in the same nucleus 
may be formed by both methods or even by a third. From these 
facts the conclusion seems inevitable that all the types of tetrad- 
formation mentioned above are merely modifications of the same 
process and have no significance in themselves so long as'a com- 
mon result is obtained. 
It seems remarkable that such obvious structures as tetrads 
should have been hitherto overlooked in the plant reproductive cells. 
Many observers have noticed that the mitosis in the sporocyte 
differs from that of all other cells, whether somatic or archesporial. 
This difference was early recognized by Guignard and Strasburger. 
Overton and Belajeff also were struck by the peculiar shape and 
appearance of the chromosomes in this mitosis; the latter especi- 
ally described them as agreeing in all particulars with Flemming’s 
account of heterotypical mitosis. Finally Farmer (1895), in speak- 
ing of the pollen-mother-cell of Lilium Martagon, refers to 
