[i STRUCTURE OF OVUM 563 



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 dis- 

 covered, 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 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. 



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 give our attention. 



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. 470) and nearly all mammals 

 (vivipa rous 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. 147) of a more or less globular 

 mass of protoplasm, spoken of as the vitellus, 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 sometimes called the germinal vesicle. 



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