On the Rhizocephalan Genus Thompsonia, etc. 23 



sacs, then, evidently originate from the internal club-shaped processes 

 of the root system; the rootlets which supplied the old external sacs do 

 not regenerate new ones at once. 



Clearly, then, successive crops of sacs containing the reproductive 

 organs are produced by the root system. It would seem likely that 

 the period of development of the parasite has been adjusted to corre- 

 spond to the time elapsing between moults of the host. This theory 

 is supported by the fact that in the case cited above the external sacs 

 contained advanced larvae at the time of moulting. I imagine that the 

 disturbance connected with capture and change of conditions hastened 

 the moult, which otherwise might not have occurred until the larvae 

 were liberated. If the correspondence does not exist, or is only partial, 

 an immense waste must go on; for surely the moult of the host and 

 consequent disconnection from the root system will suspend the devel- 

 opment of the parasite and interfere with the mechanism for securing 

 the liberation of the larvae. 



DEVELOPMENT OF GERM CELLS IN THE ROOT SYSTEM. 



I have pointed out that the peripheral root system contains in its 

 lacunar space multitudes of minute cells, which, in the formation of 

 internal buds, migrate into the interior and form the ovary of the 

 future external sac. At first they all appear exactly similar, but it is 

 clear that only a small proportion actually become ova, the remainder 

 being interstitial cells. We do not know how the germ cells are stimu- 

 lated to simultaneous development, but whatever stimulus it is that so 

 acts is not confined to the external sacs, but is also felt throughout the 

 peripheral root system; for when this is examined in a specimen with 

 well-developed external sacs there are seen, among the cellules of the 

 lacunar space, much larger bodies corresponding in size and structure 

 with the segmenting eggs found in the external sacs. One of these is 

 figured here (text-figure 8, ov.). These eggs, for such they undoubt- 

 edly are, are not so numerous as those concentrated in the external 

 sacs, but they are widely diffused throughout the peripheral system. 

 They develop until gastrulation begins, but how much further than 

 that I do not know. They certainly do not produce larvae; yet the 

 early segmentation seems to be normal and the egg is surrounded by a 

 chitinous shell. In one of my preparations the egg appeared to 

 break up at a later stage and the blastomeres to separate, but this 

 was indicated only indistinctly. The existence of these bodies in the 

 root system is exceedingly interesting and the problem of their fate 

 is important. 



I have not observed whether the stages of development of the eggs 

 in the external sacs and in the root system correspond exactly. 



