36 



GENERAL EMBRYOLOGY 



in the form of minute drops or alveoli, while their walls or the 

 irregular interalveolar spaces are of the denser material. 

 Occasionally other structural relations are seen, such as the 

 granular, where the fibrous reticulum is represented by rows of 

 excessively minute granules, and the fibrillar, where the fibers 



FIG. 17. Alveolar protoplasmic structure in the egg of the sea-urchin, Toxo- 

 pneustes, one and one-half minutes after the entrance of the spermatozoon. 

 From Wilson, "Cell," X about 2000. The protoplasm consists of alveoli sur- 

 rounded by microsomes. In the middle is the centriole, surrounded by the 

 centrosphere, while radiating from it are the rays of the aster. The large and 

 small black masses are the sperm head and middle-piece. 



of the reticulum are larger, longer, and less branched than in 

 the ordinary reticulum. It is still uncertain how exactly the 

 real structure of living protoplasm is represented by its appear- 

 ance after it has been killed, in preparing it for examination. 

 It should be remembered that in living protoplasm these 

 fibrils, reticula, etc., are in all probability fluid structures of 

 greater density than the ground substance. 



The cell is far from being a simple unit, for it contains a 



