68 WILLIAM PATTEN. 



iuvagiuation cavities, which are so strikingly like the cavities 

 of a Vertebrate brain that the resemblance cannot be due to a 

 mere coincidence, or to similarity of function. Before studying 

 the development carefully, it was a great puzzle to explain how 

 the adult brain of Limulus could resemble so closely the brain 

 of Vertebrates, and yet one be solid and the other hollow. How 

 would it be possible to convert the solid brain of Limulus into 

 the hollow one of Vertebrates, without at the same time 

 destroying its resemblance to the Vertebrate type ? A careful 

 consideration indicates that we shall not have to deal with this 

 difficulty, for the brain of Limulus already contains, either 

 potentially or actually, all the important cavities of the 

 Vertebrate brain. In order to show this, let us go back and 

 consider the manner in which the brain and nerve-cord are 

 folded off from the surface. The imperfect folds of ectoderm 

 on the lateral margins of the ventral cord, and the single layer 

 of cells advancing over the cerebral hemispheres, are, I believe, 

 morphologically the same as typical double-layered folds, such 

 as the medullary folds of Vertebrates. No one doubts, for 

 instance, that the formation of the brain and spinal cord of 

 Teleosts by solid ingrowth is anything more than a modifica- 

 tion of the same process which in other Vertebrates is ex- 

 pressed in continuous folds. Either may be derived from 

 the other. In order to prove the identity of these folds in 

 Limulus with those in Vertebrates, it is necessary to show that 

 they advance over the brain, not as they do now in a typical 

 Vertebrate embryo, but as they did in the ancestral Vertebrate ; 

 or it will be sufficient if we can show that these im- 

 perfect folds in Limulus advance in such a manner 

 that, if they were perfect duplicatures, the cavities 

 they enclosed would have the same relation to one 

 another and to the brain-lobes as those in Verte- 

 brates. These requirements, that appear so formidable on 

 any other theory, can be fairly met in Limulus. It is neces- 

 sary to remember, however, that in Limulus the advancing 

 layers of cells, representing folds of ectoderm, do not enclose 

 spacious cavities between themselves and the brain; and that 



