270 THE FIFTH DAY. [CHAP. 



bulk but also serving as the mother tissue for a far 

 greater number of organs, the alterations in the indi- 

 vidual cells 1 are, till near upon the fifth day, insignifi- 

 cant. Up to this time the mesoblast may be spoken of 

 as consisting for the most part of little more than in- 

 different tissue : of nuclei imbedded in a protoplasmic 

 cell-substance. In one spot the nuclei are closely 

 packed together, and the cell-substance scanty and 

 compact; at another the nuclei are scattered about 

 with spindle-shaped masses of protoplasm attached to 

 each, and there is a large development either of inter- 

 cellular spaces or of intracellular vacuoles filled with 

 clear fluid. The protoplasm differs in various places, 

 chiefly in being more or less granular, and less or more 

 transparent, having as yet undergone but slight chemi- 

 cal transformation. Up to this epoch (with the excep- 

 tion of the early differentiated blood and muscles of the 

 muscle plates) there are no distinct tissues, and the 

 rudiments of the various organs are simply marked out 

 by greater or less condensation of the simple meso- 

 blastic substance. 



From the fifth day onwards, however, histological 

 differentiation takes place rapidly, and it soon becomes 

 possible to speak of this or that part as being composed 

 of muscular, or cartilaginous, or connective, &c. tissue. 

 It is not within the scope of the present work to treat 

 in detail of these histogenetic changes, for information 

 concerning which we would refer the reader to histolo- 

 gical treatises. We have already had occasion to refer 



1 With the exception of the cells of the middle part of the inner 

 layer of the muscle-plates, which we have seen become converted into 

 longitudinal muscles on the third day (p. 187). 



