DEVELOPMENT OF THE BPvAlN. 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 Tnarrow (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 hrain-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 em- 

 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. 



