THE VERTEBRATE ANIMAL SUBPHYLUM VERTEBRATA 407 



vessels. As a Avhole, the brain serves as the organ of coninuinication 

 between the sense organs and the body and is the coordinator of the 

 bodily activities. 



The Reproductive System. — The vertebrate reproductive system 

 shows a fairly high degree of development. The sexes are almost 

 universally separate, with the exception of some cyclostomes. The 

 distinct gonads develop to produce special germ cells. The male 

 gonads are testes, and they produce spermatozoa which are carried 

 from the gonads by the vasa deferentia. The female gonads are 

 ovaries, and they produce ova or eggs. They are carried from the 

 body by oviducts. The males of some classes possess for use in copu- 

 lation certain accessory organs which tend to insure fertilization. 

 The vertebrates which lay eggs are spoken of as being oviparous; 

 in those in which the egg is retained in the body and the embryo 

 develops there, feeding on the yolk of the egg, and is later born 

 alive, the condition is known as ovoviviparous, and in the forms in 

 which the fertilized ovum is retained in the uterus, the embryo be- 

 ing nourished by diffusion of nutriments from the blood of the par- 

 ent, the condition is said to be viviparous, and here too the young 

 are born alive. In vertebrates the possible offspring produced each 

 season by a single individual varies from one to thousands. 



Reproductive Function. — A living organism is in numerous ways 

 similar to a machine, but reproduction of new units of living mate- 

 rial by existing organisms is hardly comparable to any mechanical 

 processes known in our industries. New organisms all arise from 

 preexisting organisms of the same kind. The process of cell divi- 

 sion is the fundamental basis for all reproduction. For centuries 

 before the invention of the microscope it was commonly believed 

 that living things arose spontaneously from nonliving material, or 

 from the dead bodies of plants and animals. Certain old books 

 carry directions for the artificial generation of mice or bees. Louis 

 Pasteur did as much as anyone to discredit this idea of spontaneous 

 generation. Our present conception is that the protoplasmic sub- 

 stance of the new individual is but a continuation of the specific 

 protoplasm peculiar to an earlier individual or in sexual reproduc- 

 tion to two individuals. Therefore, under ordinary circumstances 

 the structural and physiological complexities which arise through 

 embryonic development must be generally similar to those of the 

 predecessors. 



