REPRODUCTION AND DEVELOPMENT IN CHORDATES 



In 1824, Prevost and Dumas proved that spermatozoa are essential for 

 the formation of new individuals by filtering the seminal fluid of male frogs 

 before mixing it with eggs. No new individuals were formed under such 

 conditions. Even this experiment did not establish the fact that the sperm 

 and the egg united. It was not until 1875 that the actual penetration of an 

 egg by a spermatozoon, followed by the union of the two nuclei, was observed 

 independently by Hermann Fol and Oscar Hertwig in sea urchins (p. 499). 

 When it is recalled that the Cell Theory was formulated in 1838 and 1839 

 (p. 11), the recognition by Hertwig and Fol that the spermatozoon and egg 

 were cells is not surprising. With this recognition a sound interpretation of 

 reproduction, which had baflled students for hundreds of years, was soon 

 reached. The earliest workers had lacked mechanical equipment in the form 

 of microscopes, but the improvement in lenses was not all that was needed. 

 Progress in science is alwavs dependent on what scientists think about the 

 facts they observe. The conception of cells as the units of structure and 

 function was, and is, as great a scientific tool as the microscope, and we 

 are strikingly impressed with this fact in a study of reproduction and 

 development. 



Methods of Reproduction. Reproduction, or the formation of a new 

 individual, is accomplished in several difTerent ways. On the one hand, it 

 may occur without the production of germ cells and be the result of the 

 activity of only one individual — asexual or uniparental reproduction. On the 

 other hand, germ cells, or gametes, may be produced by two individuals and 

 unite in pairs — sexual or biparental reproduction. Among the unicellular ani- 

 mals asexual reproduction is brought about by cell division (p. 38). If this 

 cell division produces two equal cells, it is known as fission; if the two cells 

 are unequal in size, the process is called budding. Sometimes a process of 

 multiple cell division or multiple fission occurs, with the result that many 

 new individuals are produced at the same time (p. 245). Some of the 

 simpler multicellular animals, such as the coelenterates, reproduce asexually 

 by the methods of budding and strobilization; the flat worms undergo fis- 

 sion (pp. 281, 299, 305, and 322). Sexual reproduction occurs throughout 

 the Animal Kingdom by the method of syngamy, or union of two gametes to 

 form a zygote. When the gametes are differentiated into microgametes and 

 macrogametes, the process of syngamy is known as fertilization. Among the 

 protozoans both isogametes and anisogametes occur, but anisogametes are 

 typical of multicellular animals. Syngamy usually takes place in animals 

 that produce anisogametes, but parthenogenesis, or development of a macro- 

 gamete without union with a microgamete, sometimes occurs, notably among 

 the rotifers and insects (pp. 346, 472, and 535). 



Reproduction is typically a function of adult animals; germ cells are pro- 

 duced by mature individuals. However, in one of the amphibians, the 

 axolotl, immature or larval animals give rise to germ cells which function in 

 reproduction. Reproduction by immature animals is called pedogenesis and is 

 known to occur in several invertebrate groups (p. 330). 



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