410 TEXTBOOK OF ZOOLOGY 



or vice versa but tlie necessary materials are allowed to diffuse 

 through the tissue of the placenta in which both systems are dis- 

 tributed. 



Parthenogenesis. — In some species of invertebrates, sexual re- 

 production may lapse for considerable periods of time, during which 

 period no males are developed. The female produces ova which 

 develop into new individuals like herself without fertilization for 

 a whole season. This is known as parthenogenesis. Usually in the 

 fall of the year males are developed, and fertile eggs, provided with 

 protective hard shells, are produced by the females of this generation 

 to live through the winter. After winter is over such fertile eggs 

 hatch into parthenogenetic females for the next season. This process 

 is common in many smaller Crustacea, aphids, scale insects, some ants, 

 bees, wasps, thrips, a few moths, and rotifers. Artificial partheno- 

 genesis may be induced in many mature eggs by change of osmotic 

 pressure due to change of salt content in the surrounding medium. 

 Fatty acids, saponin, solanin, bile salts, benzol, toluol, chloroform, 

 ether, and alcohol are other substances which will induce it. Electric 

 stimulus, mechanical pricking, and change of temperature are also 

 used. Such methods have produced artificial parthenogenesis in eggs 

 of sea urchins, starfish, molluscs, annelids, moths, and frogs. The 

 immediate cause of the development by an egg thus stimulated is not 

 known. 



In normal fertilization of an egg by only one spermatozoon, it has 

 been found that the rate of oxidation then increases from 400 to 

 600 per cent. There are indications that this is also the case in 

 artificial parthenogenesis. This oxidation may be the cause of the 

 development in the ovum. Fertilization, where it occurs, has a dual 

 function: (1) that of stimulating the egg to develop, and (2) that 

 of introducing the properties of the male parent. 



Classification 



In most recent classifications this subphylum is divided into seven 

 classes ; however, the second is sometimes found as a subclass under 

 the third. These classes are as follows: 



Cyclostomata (si klo sto'ma ta, circle and mouth). Round-mouthed 

 fish with only median fins, unsegmented notochord, and jawless. 

 Lampreys and Hagfish. 



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