DEVELOPMRNT OP AMPHIURA SQUAIMATA. 139 



the primitive germ-cells actually migrate, and I should be 

 inclined to suggest that their place of proliferation continues 

 to be where it is undoubtedly at first, viz. in the base of the 

 ovoid gland. This wandering of the Urkeimzellen is also 

 suggested by the observation which I have made, that when the 

 rachis is complete there is great variation in the position of the 

 primitive germ-cells. Sometimes there are a large number 

 close to the ovoid gland and sometimes very few, and once just 

 close to it the aboral sinus was quite empty. 



The last stage in the development of the ovoid gland is 

 shown in fig. 11. Here we see how the axial sinus, sinus h, 

 extends laterally so as to enwrap the gland. This is a con- 

 dition of things which seems to occur sometimes sooner and 

 sometimes later. 



To demonstrate this, clear and accurate longitudinal sections 

 are required, but indeed this holds for all these sinuses. Very 

 often, in spite of all precautions, one finds their lumen quite 

 obliterated, but when one succeeds in preventing this their 

 relations are very evident. 



Before leaving the discussion of fig. Ill wish to call atten- 

 tion to the curious space sk. This I have called a shrink- 

 age space, because it is due to the different behaviour of 

 the great interradial muscle, mi., to the alcohol and fixing 

 reagents from that of the neighbouring tissues. I mention it 

 specially because I have reason to believe that it has been mis- 

 taken for part of the axial sinus by Hamann (7), and given rise 

 to his idea that the axial sinus communicates with the sinus lying 

 dorsal to the nerve-cord, a communication of which I can find no 

 trace in any of my sections. Of course it may be different in 

 the species which Hamann examined. This shrinkage space 

 is quite easy to distinguish from the various sinuses described, 

 as it has no endothelial membrane for its boundary as they 

 all have. 



Fig. 8 is a section from an individual slightly younger than 

 the one from which fig. 11 is taken ; it is interesting as show- 

 ing the opening of the ampulla, both into the pore canal and 

 into the stone canal ; it also shows very distinctly the strong 



