72 PUS-CORPUSCLES. 



The low grade of development held by the class of cells now 

 under consideration, in which those elementary formations re- 

 tain their greatest degree of individuality, is indicated by the 

 fact that it presents so very few modifications. The mucus-, 

 pus-, and lymph-corpuscles are small round cells with a nucleus 

 attached to their walls. According to Henle, mucus- and 

 pus-corpuscles cannot be distinguished in any way from one 

 another, and those of lymph differ from them only inasmuch 

 as their nucleus is more round and granulous, and does not 

 crumble under the action of acetic acid. No difference exists 

 between them in the form of the entire cell. The blood-cor- 

 puscles present a higher degree of development in this class. 

 In them we not only find very characteristic cell-contents, 

 the red colouring matter, but the form of the cell also under- 

 goes an important alteration, inasmuch as it becomes flattened. 

 As this flattening takes place in cells which float free in a 

 fluid, it cannot be explained as the result of mechanical causes, 

 bat must manifestly be regarded as a peculiar stage of deve- 

 lopment of these cells. The nucleus is persistent in all these 

 cells, whilst in those more highly developed it usually disappears 

 at some subsequent period. Throughout this class the cyto- 

 blastema is a fluid ; and it is present in greater quantity than 

 we shall find to be the case in the next class. If the egg be 

 included in this class, we have yet another peculiarity in the 

 cells to be added to the above; viz. that not only have the 

 separate yelk-cells cell-contents consisting of distinct granules, 

 but that the development of the yelk-cells within the yelk 

 considered as one cell, is a formation of cells within cells, and 

 in some of these cells even a second enclosure takes place. 

 This peculiarity, however, is one which may almost be said to 

 stand in inverse ratio to the importance of the tissue. It 

 is most frequent, perhaps indeed universal, in vegetables, 

 occurs more rarely in animals, as in the egg } crystalline lens, 

 cartilage, and so on, and appears to be altogether absent in 

 the higher structures, as areolar tissue, muscle, &c. We 

 have already discussed the other peculiarities of the cells of the 

 egg. In the following class we shall not only find a greater 

 change in the form of the cells from flattening, but we shall 

 also become acquainted with many other different modifica- 

 tions of them. 



