1 6 BIOLOGICAL LECTURES. 



(Fig. i, d). As the egg approaches maturity, the spheres be- 

 come differentiated into two groups, the larger ones becoming 

 approximately of the same size (cf. p. 10) to form the alveolar 

 spheres and crowding together, while the smaller ones remain 

 as the microsomes and finer granules embedded in the remains 

 of the continuous substance which forms the basis of the mesh- 

 work. In one sense, therefore, the alveolar spheres and the 

 microsomes are only different stages in the same morphological 

 series, though it should be remembered that they differ chem- 

 ically as well as in size, and I do not mean to imply that the 

 one may develop into the other, and both the alveolar and 

 the fibrillar or reticular structures in these eggs are of second- 

 ary origin. If this be the case, neither of these types of struc- 

 ture can be of fundamental importance ; and I fully agree with 

 the opinion of Kolliker, which has been adopted by an increas- 

 ing number of later observers, that no universal or even general 

 formula for protoplasmic structure can be given. The evidence 

 indicates that alveolar, granular, fibrillar, and reticular structures 

 are all of secondary origin and importance, and that the ultimate 

 background of protoplasmic activity is the sensibly Iiomogeneous 

 matrix or continuous substance in which those structures appear. 

 I do not mean to say that this is the only " living" element 

 in the cell. The distinction between "living" and "lifeless," 

 between "protoplasmic" and " metaplasmic," substances is 

 exceedingly difficult to define, largely on account of our vague 

 and inconsistent use of terms, for in practice we continually use 

 the word "living" to denote various degrees of vital activity. 

 Protoplasm deprived of nuclear matter has lost, wholly or in 

 part, one of the most characteristic vital properties, namely, 

 the power of synthetic metabolism; yet we still speak of it as 

 "living," because it may for a long time perform some of the 

 other functions, manifesting irritability and contractility, and 

 showing also definite coordinations of movements (as in the 

 enucleated protozoan) ; and in like manner various structural 

 elements of the cell may be termed living in a still more 

 restricted sense. In its fullest meaning, however, the word 

 " living " implies the existence of a group of cooperating factors 

 more complex than those manifested by any one substance or 



