v THE SKELETON 291 



primitive Vertebrates (Lampreys, Elasmobranchs, Teleostomes, 

 Dipnoi, Amphibia) and the early stage of development at which 

 it appears, that the hypochord is an organ of great antiquity in 

 the Vertebrate stem, but we have no definite knowledge of its 

 ancestral significance. The fact that it does not occur in Amphioxus 

 has rendered possible the suggestion that it represents the longi- 

 tudinal groove which in this animal runs along the mid-dorsal line 

 of the pharyngeal wall. But this idea is negatived by the fact that 

 the hypochord- extends right back to the tail and is not merely a 

 pharyngeal organ, and the probability seems to be that it has come 

 down from a period in evolution long before the appearance of 

 Amphioxus. It is perhaps simplest to regard it merely as an 

 accessory notochord. 



Whereas the true notochord plays an important physiological 

 role — as the main part of the axial skeleton during early stages, 

 and as the foundation for the vertebral column of later stages — the 

 hypochord has no such justification for its persistence. It lasts only 

 for a short time and eventually breaks up and completely disappears. 



In the Amniota there is no typical hypochord developed but it 

 is possible that a thickening of the mid-dorsal endoderm which is 

 frequently found in the pharyngeal region (e.g. in the Second day 

 Fowl embryo) may represent a last vestige of it. 



SKELETAL DEVELOPMENTS OF THE CONNECTIVE TISSUE 



Whereas the notochord is derived directly from the endoderm, 

 the cartilaginous and bony components of the skeleton on the other 

 hand are modifications of the mesenchyme or connective tissue, 

 which forms a considerable proportion of the entire bulk of a typical 

 vertebrate. 



Connective tissue in its least specialized form may be 

 seen in practically any late vertebrate embryo as a reticulum or 

 spongework — a syncytial framework — of much-branched cells, the 

 processes of which are continuous from one cell to another, while the 

 meshes are occupied by a clear fluid or jelly-like matrix. Masses of 

 this tissue form a kind of packing between and around the various 

 epithelia of the body, while it also, in the form of discrete wandering 

 cells, actually invades the epithelial tissues and colonizes them. 

 Such immigrant elements are found for example between the 

 muscle-fibres, in the substance of the central nervous system, and 

 even frequently between the epithelial cells of the epidermis. 



The primitive or embryonic connective tissue undergoes gradual 

 differentiation in accordance with the physiological role which it has 

 to play in different localities. This differentiation finds expression in 

 such superficial features as shape and arrangement of the individual 

 cells and more fundamentally in the peculiarities in metabolism 

 which lead to its storing up particular substances in its protoplasm 



