CHAPTER XI 

 EVIDENCES FROM EMBRYOLOGY 



THE FACTS OF REPRODUCTION AND DEVELOPMENT 



[It is now definitely known that all living creatures are mortal, at 

 least as individuals, but they all have the capacity of continuing their 

 life by the reproduction of offspring. This physical immortality is 

 based upon an actual transmission from parent to offspring of some 

 material substance which is so organized chemically as to be fully 

 representative of the race or stock to which the parent belongs. 



Reproduction may be asexual or sexual. In asexual development 

 a new individual may be produced by a process oi fission (dividing the 

 parent into two or more parts, each of which has the capacity to 

 develop into a whole new individual) ; by budding (the production of 

 new individuals by means of outgrowths of the parent-body) ; or by 

 giving off spores or eggs capable of development without fertiliza- 

 tion (parthenogenesis). In sexual reproduction two kinds of parent- 

 individuals exist: one a female which is capable of giving off relatively 

 large single cells, called eggs (ova) ; and the other a male, which is 

 capable of producing minute, usually motile cells, called spermatozoa. 

 A union of ovum and spermatozoon is usually necessary before the 

 ovum can begin its development. It is the sexual method of repro- 

 duction that will chiefly concern us here, and, for present purposes, we 

 may omit any further mention of the various asexual methods. 



An ovum may be conceived of as an individual of some definite 

 species or race reduced to the very lowest terms. It exhibits the 

 characteristic cell structure, consisting of cytoplasm and nucleus, cell 

 membrane, nuclear membrane, usually a centrosome (Fig. 43). 

 Further details as to the minute structure of the nucleus are given in 

 chapter xxvii, where the mechanism of Mendelian heredity is dealt 

 with. — Ed.] 



''The reproductive cells from the two sexes," says Wright,' ''have 

 very different appearances. In mammals, the ovum is a relatively 

 large, spherical cell, just visible to the naked eye. 



^ From Sewall Wright, Principles of Livestock Breeding, United States Depart- 

 ment of Agriculture, Bulletin No. 905. 



164 



