MORPHOLOGY OF BRAIN AND SENSE ORGANS OF LIMULUS. 31 



communicate with the exterior, or could function as sensory 

 cells, although there is every reason to believe that they are 

 derived from what were once sensory cells, like those in the 

 eyes. 



The definitive olfactory organ does not appear till the 

 changes just described are nearly finished, i. e. in the larvse 

 from one half to one inch in length. During this period buds 

 begin to appear in the ectoderm between what is left of the 

 primitive olfactory thickenings, or between what now consti- 

 tute the swollen ends of the lateral olfactory nerves. At about 

 the same time the median olfactory nerve arises from 

 the median eye-tube, near where it joins the brain as 

 an outgrowth from its anterior wall. Now this part of 

 the tube may be considered morphologically as a part of the 

 anterior neural wall or roof of the brain, just as a correspond- 

 ing part of the stalk of the pineal eye in Vertebrates or in scor- 

 pions might be regarded as a part of the brain roof (compare 

 PL 3, figs. 41 and 42). Moreover, as in their earliest stages 

 the median olfactory nerves contain numerous small ganglion- 

 cells which soon develop into two large irregular botryoidal 

 lobes, having the identical histological structure as the cere- 

 bral hemispheres ; and as the nerve soon unites directly with 

 the cerebral hemispheres, from which it then appears to be a 

 direct outgrowth, I think we are justified in regarding 

 the median olfactory nerves and olfactory lobes as 

 outgrowths from the anterior wall or roof of the cere- 

 bral hemispheres. 



Of course the fully developed lobes, as seen in fig. 17, do not 

 grow out from the cerebral hemispheres, but as the few cells 

 from which they arise do so, it amounts to the same thing. It is 

 evident that if the growth of the lobes and their separation 

 from the brain took place at the same time the olfactory lobes 

 would, as in most Vertebrates, make their appearance as massive 

 outgrowths of the cerebral hemispheres. 



The olfactory lobes vary greatly in size in diflFerent indi- 

 viduals. They are relatively largest in immature forms from 

 about 3 to 6 inches long. They may consist of two very 



