THE CEREBRAL GROOVES. 63 



transformed into parts of the supracesophageal ganglia. Struc- 

 tures of a similar nature are found in other animals. They 

 have been described by Kowalevsky in the embryo of 

 Dentalium (No. 32), in which animal the cerebral ganglia 

 are formed from the walls of two invaginations of ectoderm 

 at the apical pole of the body. They persist for some time, 

 and then vanish. In Sipunculus Spengel (No. 52) has de- 

 scribed a single canal leading from the cerebral ganglion to 

 open at the base of a tentacle. In Nemertines, as is well known > 

 canals opening on the surface penetrate into the cerebral 

 ganglia, and in Balanoglossus a single canal traverses part of 

 the central nervous system. Whether these canals leading to 

 this important part of the central nervous system are homolo- 

 gous it is difficult to say. Probably they are not, but are 

 simply analogous, their function being, or having been, in 

 some aquatic ancestor^ respiratory. I have already (No. 48) 

 suggested this as the probable genetic explanation of the 

 central canal of the Vertebrate nervous system. 



Summary of the Early Development of Nervous System. — The 

 lateral thickenings are from their origin continuous from 

 somite to somite. They begin in front of the mouth, where 

 they are connected with one another across the middle line, 

 and they end behind in the ectoderm of the primitive streak. 



The rounded elements which give rise to the nervous system 

 are derived from the ventral parts of these thickenings. They 

 are formed first in the pre-oral region, and then in the lateral 

 cords ; that is to say, the nervous system at its very first ap- 

 pearance is a continuous structure beginning in front of the 

 mouth, where it is continuous across the middle line, and ex- 

 tending backwards on each side of the mouth. 



The portions of these two cords in front of the mouth become 

 the cerebral ganglia, which give rise directly to the eyes and 

 tentacular nerves ; the portions around the mouth become the 

 circumoral commissures, while the portions behind the mouth 

 are the rudiments of the ventral nerve-cords of the adult. The 

 ectoderm from which the rounded elements arise remains 

 thickened, and give rise to structures which have been called 



