514 Origin and Descent of the Human Brain. [July, 
can as well develop into muscle, cartilage, cuticle or bone. It is 
the position of the cell and its environment which in embryology, 
as well as in phylogeny, determines what the cell shall become. 
The unity of the forces at work in nature are very evident to the 
biologist as well as to the physicist. But we must pass on to the 
main subject with the statement that after an orderly method of 
aggregation, certain protoplasmic cells arrange themselves along 
the dorsum of the embryo in the egg, and a spinal cord is formed. 
The simplest spinal cord is owned by the Amphioxus, a verte- 
brate lower than the lamprey of our lakes. This fish-like animal 
has no brain. Extended the length of the body, is the cord, and 
nerves enter it dorsally and ventrally ; the second pair of nerves 
of the head end pass caudally. Those along the back in this 
diagram are sensory, the lowermost being motor : 
Owen compares these longer nerves to the nervus lateralis of 
the cod. He mentions them also as nerves of association com- 
parable to the trigeminal and vagal. : 
The cord of the lamprey (Petromyzon fluviatilis ) is quite rudi- 
mentary, but a distinct brain presents itself in this case for analy- 
sis. We find certain intumescences attached to the spinal cord at 
the head end, which can be represented schematically thus: 
The real appearance of these ganglionic swellings, for such 
they are, resembles the embryonic fusion of cerebral and spit 
ganglia. A very important revelation concerning the homologic® 
of these tubercles, I hope to be able to present to the next nie 
ing of the American Neurological Society. 
Notice that in this low vertebrate form, these enlargements on 
the sensory or ingoing nerves, occur at the head. : 
