The Prophase in the Ovigenesis and the Sperinatogenesis of Planaria etc. 437 
At the stage illustrated in Fig. 4, PI. I, the spireme is definitelv 
f'ormed, approximately of the same thickuess throughout and evenly 
chromatic. From this point up to the middle of the synapsis (Figs. 4. 
21, 22 and 5) we are dealiug obviously with leptotene uuclei, like 
tliose described by Gregoire in AUium and Osmunda. The longi- 
tudinal pairing of the univalent spireme segments is completed, 
Gregoire contends, at about the middle of the synapsis, giviug rise 
to the thick spireme or pachynema. 
Before reviewing what occurs in Planaria during these phases, 
it is necessary to point out a serious gap in Gregoire’s work ('07). 
In no part of his paper do we learn exaetly how the univalent 
spireme threads pair up in the zygotene nuclei. 
On pagc 377, referring to his tigures for AUium fisitulosum he 
says. „La fig. 45 represente des tilaments miuces qui, comme le 
montre la serie des figs. 37, 38, 40, 42 et 43 vienuent a peine de se 
differencir aux depens du reseau chromatique et qui entrent en con- 
traction; plusieurs d'entre eux sont clairement conjugues deux par 
deux. Dans le fig. 46 et 47 (zygotene nuclei) un peu plus avancee 
et a noyau plus contracte, l’assoeiation par paires est accomplie 
pour tous les filaments.” 
From this one might gather that the pairing Starts verv early 
in the leptotene stage and is finished in the zygotene. But this is 
not so, for on page 390, he expressly says of Figs. 42, 43 and 44, 
which ,,offrent des parallelismes entre certains de leur filaments. Nous 
pensons cependant que ce n'est pas la le debut de Laccolement chromo- 
somique, mais un resultat de la transformation du reseau en tilaments”. 
Evidently then, the pairing takes place wholly in the zytogene 
stage, and it is exactlv in this stage in which Gregoire fails to show 
the modus operandi of the pairing. 
In my Figs. 21 and 22, it will be seen that one or two threads 
lie close together and parallel, but only for a short distance, and such 
parallelism is entirely accidental. 
The conclusion to be drawn from the series of Figs. 20, 4, 21, 
22, 5, 6 and 7, is that the spireme is at first very long and thin and 
tills up the whole nucleus; euch segment of the spireme slowly con- 
denses and shortens, but at the same time the nucleus and the nuclear 
sap is continually increasing, so that eventually the spireme fails to 
tili up the whole nuclear cavity which has become much too large 
for it (Fig. 7). Therefore, the synaptic figure is the result, not 
of contraction, but condensation. 
