997 
finally their development from both muscle-cells and cells of the ge- 
neral parenchyma, all these seem to indicate that some complex of 
stimuli or conditions exists in the testis-zone which determines the 
direction of development of whatever cells may be affected by it in so 
far as these cells are capable of reacting to it. Apparently in this 
case the somatic differentiation which these cells have undergone has not 
been sufficient to prevent their reaction to the altered conditions. It 
does not follow, however, that all somatic cells in other species are 
capable of giving rise to germ cells. Doubtless many of them have 
been altered so greatly by the conditions to which they have been 
subjected that they are incapable of any such reaction. 
The part played by amitosis in the development of the testes is 
also of considerable theoretical importance. A brief account of it has 
already been given in the paper referred to above and it will form 
the subject of a more extended account to appear later. That the 
germ cells in Moniezia arise chiefly by amitotic division cannot be 
doubted. But mitosis occurs occasionally as noted in Fig. 3 and the 
relative frequency of the two forms of division differs in different 
chains, proglottids and regions. 
Hull Zoological Laboratory, University of Chicago, September, 1906. 
Nachdruck verboten. 
Ricerche istologiche e citologiche sul timo degli Uecelli. 
Pel Dr. Carmeto Craccıo. 
Con 3 figure. 
Il timo degli Uccelli &€ un organo persistente per tutta la vita, sia 
nella sua parte linfoide, sia nella sua parte epiteliale, mentre nei 
Mammiferi nel periodo di vita extra-uterino esso subisce un processo 
d’involuzione per cui sparisce la parte linfoide e persiste invece, se- 
condo alcuni autori, semplicemente la parte epiteliale. — Io, avendo 
intrapreso delle ricerche sul timo degli Uccelli, ho di mira principal- 
mente di portare luce sulla parte epiteliale di quest’ organo, dimo- 
strando come essa rappresenta un organo endocrino e attivamente 
funzionante. — E solo dunque da questo punto di vista che io me ne 
interesserd, lasciando da parte, almeno per ora, cid che riguarda lo 
sviluppo e l’origine come organo linfoide ed epiteliale. 
Materiale e metodi di ricerca: Le mie ricerche sono state fatte 
sul pollo e sul colombo; la tecnica usata é stata la seguente: 
