546 STRUCTURE OF OVUM chap. 



constituent cells of the higher animals and plants were dis- 

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

 Schleiden and Schwann, the)' were looked upon as the 

 iilthna Thtik of microscopic analysis. Now the demonstra- 

 tion 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 con- 

 jecture. But of one thing we may feel confident — of the 

 great strides which our knowledge of the constitution 

 of living things is destined to make during the next half 

 century. 



Structure of the ovum. The striking general resemblance 

 between the cells of the higher animals 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 earth- 

 worms, crayfishes, frogs, birds, &c. {oviparous forms), or 

 undergoes development within the body of the parent, as 

 in some dogfishes (p. 454) and nearly all mammals 

 {viviparous forms). 



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

 in all animals from the highest to the lowest (compare 

 p. 195). It consists (Fig. 137) of a more or less globular 

 mass of protoplasm, in which are deposited particles of a 

 proteinaceous substance known as yolk-granules. Within 

 the protoplasm is a large nucleus containing chromatin 

 as well as one or more nucleoli — which are often known as 

 germinal spots, the entire nucleus of the ovum being called 



