REPRODUCTION 43 



volume of the smallest spermatozoon should be compared to that of the 

 ovum of a moa or ^Epyornis. 



It has been stated above that among birds there is some correlation 

 between the size of the egg and the size of the bird. This is a necessary 

 consequence of the fact that in all birds the yolk must provide the material 

 needed to carry development up to the point of hatching, at which time 

 the young birds have attained roughly corresponding stages in develop- 

 ment and growth. When, however, animals of different classes and eggs 

 of different types are brought into comparison, there appears a striking 

 lack of any correlation between size of animal and size of egg. The eggs 

 of some small salamanders are vastly larger than the eggs of some large 

 fishes. From the reasonable assumption that the eggs of elephants and 

 whales are of the typical mammalian sort, it follows that the egg of a 

 humming-bird is tremendously larger than that of one of these gigantic 

 mammals. Egg size is correlated primarily with the method of develop- 

 ment. Correlation with body size appears only when the developmental 

 history of the animals is similar. 



Fertilization 



The event which immediately initiates the development of a new 

 individual is the "fertilization" of the egg by the spermatozoon. The 

 motile sperm cell penetrates the egg and the sperm "head," a compact 

 mass of nuclear substance (chromatin), becomes joined with the chromatin 

 of the egg nucleus. In consequence of the so-called " maturation" process 

 through which all germ cells pass, each mature ovum and spermatozoon 

 contains only the "haploid" complex of chromosomes. Therefore the 

 junction of the sperm chromatin and the egg chromatin provides the 

 fertiUzed egg with a nucleus possessing the "diploid" complex of chro- 

 matin — that is, the full complement characteristic of the nuclei of all 

 the body cells of the animal. 



So far as known, the cytoplasmic part of the spermatozoon contributes 

 to the functional ovum nothing except, possibly, a single centrosome. 

 However that may be, it is certainly true that the fertilized egg, although 

 it is the product of two cells, possesses the complete mechanism character- 

 istic of a single cell, and it does not visibly possess anything more than 

 that. Its yolk is something characteristic of eggs, but yolk is an inert 

 food substance, not a mechanism. The fertilized egg contains no visible 

 structures which would adequately account for its development into a 

 large complex animal possessing the specific form and characteristics 

 of the parent animals. In fact, as compared with such specialized cells 

 as those of muscle and nervous tissue, the egg cell is strikingly devoid of 

 visible special mechanism. 



