DEVELOPMENT OF THE BRAIN, 303 
the form of a longitudinal fold, and both folds then grow 
together over the furrow in the central line, and thus form 
a cylindrical tube. This tube is called the marrow-tube, or 
medullary canal, because it is the foundation of the central 
nervous system, the spinal marrow (medulla spinalis). At 
first it is pointed both in front and behind, and it remains so 
for life in the lowest vertebrate animal, the brainless, skull- 
less Lancelet (Amphioxus). But in all other vertebrate 
animals, which we distinguish from the latter as skulled 
animals, or Craniota, a difference between the fore and 
hinder end of the marrow tube soon becomes visible, the 
fore end becoming dilated, and changing into a roundish 
bladder, the foundation of the brain. 
In all Craniota, that is, in all vertebrate animals possess- 
ing skull and brain, the brain, which is at first only the 
bladder-shaped dilatation of the anterior end of the spinal 
marrow, divides into five bladders lying one behind the 
other, four superficial, transverse in-nippings being formed. 
These five brain-bladders, out of which afterwards arise all 
the different parts of the intricately constructed brain, can 
be seen in their original condition in the embryo represented 
in Fig. 7. It is just the same whether we examine the ein- 
bryo of a dog, a fowl, a lizard, or any other higher vertebrate 
animal. For the embryos of the different skulled animals 
(at least the three higher classes of them, the reptiles, birds 
and mammals) cannot be in any way distinguished at the 
stage represented in Fig. 7. The whole form of the body is 
as yet exceedingly simple, being merely a thin, leaf-like disc. 
Face, legs, intestines, etc., are as yet completely wanting. 
But the five bladders are already quite distinct from one 
another. 
