520 RECORD OF SCIENCE FOR 1886. 



others furnished by the auimal kingdom, he has enunciated some rather 

 curious propositions to which he was led. 



According to Dr. von Ihering, " in all groups twins may occur from 

 one ovum; the polar bodies are morphologically nothing less than 

 abortive germs; in fact, the origin of the multiple embryos from a sin- 

 gle ovum is the primitive condition ; the development of only one is 

 the secondary and adaptive. Now if this be pressed to its logical con- 

 clusion," continues Dr. von Ihering, ^' one would be forced to the para- 

 doxical conclusion that the Praopus, for instance, brings forth grand- 

 children, and that the mother of twins from one ovum is really their 

 grandmother." If this be so, the categories previously admitted in the 

 schemes of development are evidently insufQcient, and Dr. von Ihering 

 has proposed a revised classification, as follows : 



I. "Hologenous Development (Haeckels' hypogeuesis). The fertil- 

 ized ovum develops with or without metamorphosis into a single indi- 

 vidual. 



II. " Merogenous Development. The fertilized ovum develops into 

 tioo or more individuals, which 



A. "return directly to the parent form and mode of reproduction 

 {Temnogenesis), or, 



B. " exhibit an antithesis of diversely reproducing individuals or gen- 

 erations (Metagenesis, or Alternation of Generations) 



{a) '• Calycogenesis in Salpce and Medusae, 



(b) " Pcedogenesis in Cecidomyce, 



(c) '■^ Eeterogenesis, in which either both generations reproduce sex- 

 ually or one or several multiply parthenogenetically ." (Biol. Central bl., 

 VI, pp. 532-539; J. R. M. S. (2), vn, p. 44.) 



The species of Manatee. — A few years ago a species of Manatee was 

 described as peculiar to the headwaters of the Amazon. ThOiSpeci- 

 mens on which the species was based were obtained in the early part 

 of the century by Dr. Natterer, but for many years it had remained unde- 

 scribed. This species and the other representatives of the genus have 

 been recently investigated by Dr. 0. Uartlaub. The result of Dr. llart- 

 laub's examination of all the specimens accessible to him of the family of 

 Manatees is a confirmation of the species indicated, Manatus (rather 

 Trichechiis) immguis and the ascertaining of good characters. Two 

 other of the formerly recognized species are admitted, and for them tho 

 names scnegalcnsis and latirostris are retained. The former, as is of 

 course indicated by the name, is of African origin, and the latter oc- 

 curs in the West Indies and along the Florida coast. For the details 

 of the difi'erences between the various species, reference must be made 

 to the memoir of Dr. Hartlaub in the " Zoologische Jahrbiicher." 

 Here it can only be stated that the differences are well marked in the 

 young as well as in the adult. Some of the most salient are the differ- 

 ences in the width and extension of the faeial bones, the character of 



