Chap. VII.] THE GENESIS OF TISSUES AND ORGANS. 119 



formed during early embryonic life in the liver, but subsequently 

 in the spleen, and lymphatic glands. 



Connective Tissue. The mesoblast cells between the develop- 

 ing blood-vessels become somewhat separated from one another, 

 but at the same time united by processes passing from one to the 

 other. In the interspaces there collects an albuminous fluid, 

 which will be converted into the ground substance of the tissue. 

 Whether the fibrillated bundles of white fibrous tissue are (1) 

 separated off from the outer portions of the cells, the residue of the 

 cell remaining as a connective tissue corpuscle ; or (2) developed 

 in the intercellular ground substance, the cells remaining as the 

 corpuscles, is still undetermined. It is probable that elastic fibres 

 are formed in the intercellular substance between fusiform cells. 



Nerve. The mode of development of nerve-fibres and 

 nerve-cells requires further investigation. It is now generally 

 believed, however, that all the nerves arise as outgrowths of 

 epiblast cells from the central nervous system. These cells 

 elongate and arrange themselves end to end with the consequent 

 production of axis fibres. If during life an axis fibre is cut, the 

 part furthest from the central nervous system dies and decays, 

 and is replaced by the outgrowth of the cut end in living con- 

 nection with the central system. 



The medullary sheath is said to be formed from mesoblast. 

 The cells are said to develop around the axis fibres. Each inter- 

 node is supposed to be formed from a single cell, the greater 

 part of which becomes converted into fatty matter, the nucleus 

 and residuary protoplasm remaining as the nerve-corpuscle 

 (see p. 74) The primitive sheath is regarded as the cell mem- 

 brane of the medullary cell. 



Muscle. Unstriped muscular fibres are developed from meso- 

 blastic cells (for the most part from the splanchnic layer of the 

 lateral plates), which become elongated, fusiform, and flattened. 



Striped muscular fibres, notwithstanding their extreme length 

 (up to 2 inches), would also seem to be derived from single cells. 

 At first oval, the fibre becomes an elongated spindle in which the 



