CHAPTER 2 

 REPRODUCTION 



Anatomy, broadly defined, includes embryology which deals with the 

 progressively changing anatomy of the animal in the course of its develop- 

 ment from egg to adult. Many anatomical peculiarities of animals are 

 unintelligible so long as only the adult is studied. Embryology gives 

 some reason for such facts as that the chief artery emerging from the 

 heart turns to the right in a bird but to the left in a mammal and that the 

 diaphragm of a mammal is supplied by nerves from the neck region instead 

 of from the neighboring trunk region of the spinal cord. The theory 

 of evolution rests to an important extent on facts derived from the com- 

 parative embryology of vertebrates. 



Sexes. Reproduction in the vertebrates always involves gonads 

 of two types, the ovary which produces eggs (ova) and the testis which 

 produces sperm (spermatozoa). In some tunicates (Urochorda), pre- 

 sumably remote allies of vertebrates, alternation of sexual and asexual 

 generations occurs. A fertilized egg becomes an asexual individual 

 from which arise buds. These become sexual adults which are her- 

 maphrodite, that is, they produce both eggs and sperm. 



In all vertebrates except a few fishes the individual is either male or 

 female — the dioecious condition. The eel-like hag, Myxine (a cyclo- 

 stome), and several of the bony fishes (Teleostei) are normally her- 

 maphrodite (monoecious). Among vertebrates which are normally 

 dioecious many abnormal cases have been reported, especially in fishes 

 and amphibians, in which germ cells of both sexes were found in one 

 individual. 



The Germinal Bodies. The spermatozoa are derived from cells 

 in the walls of delicate tubules which are the essential part of the testis. 

 The ova come from primordial germ cells contained within the tissues 

 of the usually solid ovary. (Fig. 274) 



The "head" of the spermatozoon (Fig. 25) consists of compacted 

 nuclear material (chromatin) derived from the primordial germ cell. 

 A locomotor "tail" is formed from the cytoplasm (extranuclear proto- 

 plasm) of the original cell. The "ripe" spermatozoon is essentially a 

 motile nucleus. 



The egg in the course of its differentiation acquires a greatly increased 

 body of cytoplasm within which is deposited more or less food material, 



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