68 COMPLEXITY OF CELL STRUCTURE less. 



the cells themselves is an easy matter, the problem is to 

 make out their ultimate constitution. What would be the 

 result if we could get microscopes as superior to those of 

 to-day as those of to-day are to the primitive instruments of 

 eighty or ninety years ago, it is impossible even to conjecture. 

 But of one thing we may feel confident — of the enormous 

 strides which our knowledge of the constitution of living 

 things is destined to make during the next half century. 



The striking general resemblance between the cells of the 

 higher animals and plants and entire unicellular organisms 

 has been commented on as a very remarkable fact : there is 

 another equally significant circumstance to which we must 

 now advert. 



All the higher animals begin life as an egg, which is either 

 passed out of the body of the parent as such, as in most 

 fishes, frogs, birds, &c., or undergoes the first stages of its 

 development within the body of the parent, as in sharks, 

 some reptiles, and nearly all mammals. 



The structure of the egg is, in essential respects, the same 

 in all animals from the highest to the lowest. In a jelly-fish, 

 for instance, it consists (Fig. ii, a) of a globular mass of 

 protoplasm {gd), in which are deposited granules of a pro- 

 teinaceous substance known as yolk-spherules. Within the 

 protoplasm is a large clear nucleus {g.v) the chromatin of 

 which is aggregated into a central mass or nucleolus ig.m). 

 An investing membrane may or may not be present. In 

 other words the egg is a cell : it is convenient, for reasons 

 which will appear immediately, to speak of it as the ovum 

 or egg-cell. 



The young or immature ova of all animals present this 

 structure, but in many cases certain modifications are under- 

 gone before the egg is mature, i.e.., capable of development 



