CHAPTER 2 

 REPRODUCTION 



Anatomy deals with the structure of the organs of an animal. Struc- 

 ture is described primarily in terms of form. Form is extension in space. 

 The animal, however, exists not only in space but in time. The animal's 

 anatomy is more than the form which it possesses at a particular instant. 

 It is form which passes through a series of definite and characteristic 

 changes beginning with the development of an egg and ending at death. 

 Therefore a description of the anatomy of an adult animal is to be 

 regarded as merely a cross section of its whole anatomy. Embryology 

 deals with the life history during that early period in which the form of 

 the organism is changing rapidly, progressing from the comparative 

 simplicity of the egg to the structural complexity of the adult. 



In the anatomy of an adult vertebrate are found many things which 

 remain unintelligible so long as attention is confined to the adult stage of 

 the animal. Why does the chief artery emerging from the heart turn to 

 the right in a bird, but to the left in a mammal? Why are the major 

 blood vessels in the body of mammals, even among individuals of the 

 same species, highly variable in their arrangement? Why does the 

 human diaphragm receive its nerve supply from the neck region of the 

 spinal cord rather than from the neighboring trunk region of the cord? 

 Why does a male dogfish have an apparently useless structure which 

 looks like the anterior portion of the oviduct of the female? These 

 questions and many more of the same sort are answered by embryology. 

 Therefore it is essential that the study of the anatomy of any organ should 

 include the embryonic history of the organ. 



DIFFERENTIATION OF SEXES 



Reproduction in the vertebrates is always sexual. That is, it always 

 involves the differentiation of gonads of two types; the ovary which 

 produces eggs or ova, and the testis which produces sperm or spermatozoa. 

 However, in some members of the Urochorda (tunicates), which are 

 presumably remote chordate alUes of the vertebrates, occurs an alterna- 

 tion of sexually produced and asexually produced generations. The 

 sexual individual is hermaphrodite, that is, it possesses both ovary and 

 testis, but the eggs are usually, if not always, fertilized by sperm from 

 another individual. The fertiUzed egg develops into an adult which, 

 however, is devoid of gonads. Then this asexual individual produces 



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