6 TRANSACTIONS LIVERPOOL BIOLOGICAL SOCIETY. 



typed by the formation at a later period of the vertebral 

 column. 



Xot only the muscular and nervous systems, but other 

 organs and systems become stamped by secondary 

 characters of the same kind, notably the vascular and 

 genito-urinary systems, producing series of segmental 

 vessels and segmental tubules, all be it noted, derived 

 from the same mesoblastic germinal layer. 



At the same time it must be borne in mind that the 

 essential feature is a longitudinal series of organs, and 

 that the segmentation of the vertebrate is neither constant 

 throughout its length, nor necessarily complete at any 

 point. 



Time will not permit me to illustrate this point fully, 

 but I may be allowed to refer to one or two organs and 

 systems in passing. 



The Axial Skeleton — The vertebral segment is never 

 complete even in the skeleton. The nearest aprnoach to 

 it occurs in the thorax, but even here (in mammals) the 

 sternum is interposed to fill up the deficiences of the 

 segmental process, as a longitudinal element which com- 

 pletes the thoracic skeleton. In limbless reptiles the 

 deficiency remains, and the ribs terminate in free ends. 



The secondary nature of segmentation is illustrated 

 by the development of the spinal column in rodents. In 

 the rat at birth, when the cartilaginous elements of the 

 skeleton are defined, and the process of ossification is well 

 advanced, the bodies of the vertebra are not segmented 

 off from one another. They are merely constricted at 

 intervals, and the whole series form a kind of thick 

 tubular investment for the notochord. 



There are further certain obvious instances in which 

 segmentation is arrested: e.g., in the cervical vertebra? of 

 cetacea, and in tin 1 human sacrum. The best instance of 



