GERM CELLS OF ANURANS 273 
The twisting together of the leptotene threads to form the 
double pachytene spireme which occurs during the amphitene in 
anura (fig. 12) seems to the writer to be the period when the 
mechanical conditions for the 'chiasma-type' theory of Janssens 
are present, and not during the later stage figured by this writer. 
This theory of 'Chiasma-type' has been extensively employed by 
Professor Morgan and his co-workers to explain 'crossing-over' in 
Drosophila. It has repeatedly been observed that genetic fac- 
tors belonging to a certain group, and presumably^ carried by a 
single chromosome, go into a mating together, but do not always 
reappear together, as they should, if carried by a single chromo- 
some that has maintained its individuality throughout. Janssens 
endeavored to explain the anomalous genetic behavior of such fac- 
tors on mechanical grounds, i.e., by showing that in the behavior 
of the chromosomes, at certain stages in the maturation cycle, 
it is possible for actual 'crossing-over' of parts of homologous 
chromosomes to occur, and this exchange of parts of chromosomes 
he termed 'Chiasmatype.' This theory is based upon a study of 
certain postspireme (strepsinema) stages in the spermatogenesis 
of the urodele Batracoseps. In this form, after the secondary 
longitudinal spht (equational split) has taken place, the tetrads 
are composed of four separate strands or chromatids. These 
strands may cross each other at certain places, and, owing to 
strains or weakness at the point of contact, break, subsequently 
recombining in such a manner as to form threads composed of 
parts of both original strands. That is to say, parts of the two 
strands 'crossed over' and became incorporated as a portion of the 
opposite chromatid. Janssens has carefully figured many such 
apparent 'cross-overs' in the postspireme stages of Batracoseps. 
There can be no reasonable doubt of the accuracy of the genetic 
evidence for 'crossing-over,' nor of the general truth involved in 
the chiasma-type theory. The point to be considered here is 
whether or not the cytological evidence for this view is not more 
convincing if a stage in the maturation cycle of the chromosomes 
is used as the basis of cross-over, earlier than the early tetrad 
state employed b}- Janssens. The mechanical conditions fur- 
nished by the amphitene period in the bullfrog, for crossing over of 
