Reproduction 



7 



Animals exist not only in space but also in time. The anatomy of an 

 animal 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 in the course of development of the egg into the adult. The 

 anatomy of an adult is merely a cross section of the animal's whole 

 anatomy. Therefore anatomy, broadly defined, includes embryology. 



Many anatomic 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 comparative 

 embryology of vertebrates. 



(This chapter on reproduction contains occasional statements which 

 imply or assume that the main line of evolution of the vertebrates 

 began with fishes and proceeded via amphibians to reptiles where it 

 bifurcated to produce birds and mammals. The facts which indicate 

 this order are derived largely from comparative adult anatomy, which is 

 discussed in later chapters (Part III). If the anticipation of conclusions 

 which may be drawn from the later chapters should seem to violate the 

 logical structure of this book, the fault may be condoned on the ground 

 that appreciation of the possible significance of the embryologic facts 

 may best be assured if the significance is pointed out as the facts are 

 presented. See the Preface.) 



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) , 

 presumably remote allies of vertebrates, alternation of sexual and 

 asexual generations occurs. A fertilized egg becomes an asexual indi- 



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