148 AMPHIMIXIS OR ESSENTIAL MEANING OF [XII. 



halves of threads, representing the arms of the Ophiurid, are 

 directed to each other by the activity of the achromatin nuclear 

 network as they are moved hither and thither, it is essential for 

 them to have a central point of support, i. e. the part represent- 

 ing the body of the Ophiurid. No conclusive objection can be 

 raised against the view that the shortening process is by itself 

 sufficient to convert a long thread into a short thick rod ; for 

 we know that nuclear threads are subject to great shortening. 

 But Hertwig himself seems to have had some doubts as to the 

 validity of this explanation which he offers. In support of it 

 he reminds us of ' the considerable shortening undergone by 

 the threads in the spermatozoa of the salamander,' but he adds 

 that this amount is very far below that required in the case of 

 Ascaris if his interpretation is to be accepted. 



The bifid form of the rods indicates the longitudinal fusion 

 of two threads with their points left free, and finally the 

 position of the rods with their bases apposed, and thus 

 standing as it w^ere, back to back, is more intelligible when 

 we suppose that adjacent arms of the Ophiurid are fused 

 together, rather than that each of the long chromatin threads 

 has shortened to a rod. If the latter were true we should 

 expect that the rods would lie in the middle of the mass of 

 ' linin ' representing the Ophiurid body \ and this, according to 

 Hertwig's figures, does not seem to be the case. 



We may very properly be asked for the observations which 

 support this view of a fusion between the halves of threads. 

 So careful an observer as O. Hertwig can scarcely have 

 overlooked these stages, if they have any existence. This 

 I freely acknowledge ; but in Plate I he shows a series of 

 figures in which two arms of the Ophiurid are approaching 

 each other, and are more or less fused together. Perhaps Figs. 

 27, 28, 29 should be understood in this way, and we might then 

 conclude that the threads only begin to fuse after they have 

 already undergone considerable shortening, and further that 

 the fusion commences at the position of crossing and proceeds 



^ Because the middle of each long thread passes through the centre 

 of the ' linin,' while the gradual shortening of the two ends would 

 finally reduce the thread to this greatly thickened middle part. If ad- 

 jacent halves fused together there would be no such arrangement : 

 they would tend to radiate away from the mass of ' linin ' in which their 

 bases alone would lie. 



