536 JULIA. B. PLATT. 
problem of the embryologist, and we are wont to forget that in 
the growth we study each cause plays its part. 
Observing that the complicated nervous system of the adult 
may be traced to the embryonic neural plate, we forget to 
note in this the expression of final cause, and to consider that 
meantime, before the neural plate enters upon its co-ordinating 
functions, efficient cause must find in the egg a material 
medium of co-ordination. 
The living protoplasm of every cell is the first nervous 
system of the egg. Step by step that specialised structure 
which we recognise as a nervous system usurps the power that 
belongs originally to each cell. What wonder, then, that 
primitive nerves are cellular ? 
In a recent publication Sedgwick (34) calls attention to the 
fact that embryonic tissues are from the first connected by a 
reticulum of fine protoplasmic threads. These tiny strands of 
protoplasm connecting cell with cell are also evident in Necturus. 
They are, as development shows, associated with the first steps 
in the formation of the nervous system. When Sedgwick adds, 
however, that the cell theory is a myth, and that the reticulum 
forms a common medium into which the nuclei migrate, in- 
stancing the formation of the neural crest, 1 cannot agree with 
him. The ontogenetic changes that appear in Necturus seem 
to me to refute such a supposition. 
At an early period of development the superficial ectoderm 
lies directly on the neural tube, and, as Sedgwick states, fine 
protoplasmic threads connect the two, otherwise sharply 
bounded, tissues. Later mesenchyme appears between the 
skin and brain, or, as Sedgwick tells us, nuclei migrate from 
below into the existing reticulum. The protoplasmic threads, 
however, that at first connect the brain and superficial ectoderm 
in Necturus contain no yolk granules, while the nuclei that 
come from below advance in a surrounding reticulum heavy 
with enormous yolk granules. Evidently they did not migrate 
into the reticulum they found, but brought their own reticulum 
with them. We have now a reticulum between the brain and 
skin, composed in part, as before, of delicate fibres that arise 
