COMPLEXITY OF CELL STRUCTURE 67 



In this connection the reader will not fail to note the 

 extreme complexity of structure revealed in cells and their 

 nuclei by the highest powers of the microscope. When the 

 constituent cells of the higher animals and plants were 

 discovered, during the early years of the present century, by 

 Schleiden and Schwann, they were looked upon as the ultima 

 Thule of microscopic analysis. Now the demonstration of 

 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 

 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. 12, 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 (g. m). 



F 2 



