THE TISSUES. 29 



is very closely allied to the former kind, for its iatercellular substance 

 also is secreted from cells — arises, that is, by the differentiation of a 

 part of the protoplasm. The connection is stiU .closer if we regard 

 the earliest stage in the process. In both cases a homogeneous sub- 

 stance is secreted, which is hardened by calcareous compounds, and 

 into this the cells, which form it, send their processes. If this 

 process goes on in the same way as it began, so that a complete cell 

 never passes into the secreted layers, it leads to the formation of 

 that form of osseous tissue which is traversed by fine pore-canaliculi 

 only, arranged for the most part in parallel lines. If some of the 

 secreting cells gradually pass into the secreted substance, that 

 substance becomes an intercellular substance containing bone-cells. 



Morphological Elements of the Nutrient Fluid. 



§ 24. 



The cells, which are suspended ia the nutrient fluid of the 

 body, and which are its form-elements, are closely connected in 

 origin with the connective tissue. If it is allowable to regard this 

 fluid as an intercellular substance, then the whole of the nutrient 

 fluid might be compared to a tissue, which would not differ from 

 the other tissues of the connective series in any essential point 

 other than its fluid condition. Even if we admit it to possess 

 another function in consequence of this fluid condition, yet this 

 function must be held as falling well within the category of vege- 

 tative functions. Apart from the importance of these considerations, 

 the form-elements in question must be enumerated in the present 

 place, for they take their earliest origin from the tissue which 

 forms the walls for the vessels of the nutrient fluid. As far as its 

 characters are known, a portion of the cells which form the mesoderm 

 during the processes of division do not become connected with the 

 rest, but remain isolated in the fluid which flUs these canals or 

 spaces, which fluid is known as blood. These form-elements then 

 are the blood-cells. In the Invertebrata they appear, as a rule, in 

 the form of completely indifferent cells, 

 consisting of a nucleus (Fig. 12, n) and 

 protoplasm, which latter exhibits amoeboid 

 movements. Among the Vertebrata these 

 morphological elements persist as lymph- 

 cells in the Craniata, while in the blood- 

 fluid proper there are elements which are .„. ,„ ^, , 



j-^nn ,^ 1, ^ Fig. 12. Blood corpuscles 



derived from these forms, but are much ^f a Crustacean (Maja 

 altered. These latter have lost their amoe- Squinado) with protoplas- 

 bold character during differentiation, and ™ic processes, n Nucleus, 

 have the form of rounded or oval discs, 



the nucleus of which disappears in the Mammalia, though present 

 ia the rest of the Vertebrata. 



