62 HISTORY OF THE HUMAN BODY 



dorsal, ventral and two lateral aspects. If this metamorphosis 

 has any biogenetic value, that is, if it is indicative of a genu- 

 ine historic stage in the phylogeny of vertebrates, it suggests 

 an ancestral gastrula that sank to the bottom, lay upon its 

 side and exchanged a free swimming for a crawling mode of 

 locomotion, apical pole forward. Such an hypothetical form 

 as this corresponds, however, to nothing known at the present 

 time, but may well have disappeared without trace, since a 

 similar fate has happened to the transition forms linking 

 the vertebrates to the other Metazoa, leaving the group un- 

 usually isolated. [See Chapter XII. ] 



There now occur several simultaneous changes which inau- 

 gurate the essential vertebrate structure and are best explained 

 by the help of the accompanying diagrams. [Plates I. and 

 II.] The gastrula has now become considerably elongated in 

 the direction of the newly acquired secondary axis and is rep- 

 resented as cut transversely across, the diagram representing 

 the posterior portion and showing the cross-section as well as 

 a portion of the length. The most superficial of these changes 

 involves a longitudinal mid-dorsal stripe, which becomes grad- 

 ually turned in, forming a trough. Through the fusion in the 

 median line of the edges of the trough, the turned-in portion 

 becomes a tube, which ultimately frees itself from its attach- 

 ment to the rest of the ectoderm, and forms the neural tube, 

 the anlage* of the nervous system. The walls of this tube, by 

 an excessive thickening of certain definite portions, become the 

 brain and spinal cord, and the lumen is perpetuated as the ven- 

 tricles of the brain and the canalis centralis of the cord, the 

 embryonal communication between these cavities being re- 

 tained throughout life. 



A somewhat similar structure, also median, arises from the 

 dorsal wall of the endoderm. This appears at first as an in- 

 verted trough, and possesses a narrow lumen, but it eventually 



* The word Anlage is borrowed from the German to express a concep- 

 tion for which there is no English equivalent. It signifies the first visible 

 indication of a part that appears in the embryo, and may thus signify 

 either a definite cell-mass or a slight change in the arrangement of cells. 



