302 PHILLIPS—A REVIEW OF PARTHENOGENESIS.  [0ct.16, _ 
where it is desirable to produce individuals quickly, since here the 
larval stages are omitted. 
Unisexual and bisexual generations alternate with each other in 
various ways in Crustacea and the mode of the alternation is 
remarkably related to their environment, as has been shown by 
Weismann. According to whether the causes of destruction visit a 
colony once or several times during the year we find forms which 
have one or several cycles of parthenogenetic and bisexual genera- ~ 
tions, and finally species are known which show no alternation. 
These are designated as monocyclical, polycyclical and acyclical 
respectively. 
TREMATODES. 
The development of the cercaria and redia stages of Disto 
mum has been the subject of much discussion for a long time. — 
Leuckart, in his Parastten des Menschen, gives an historical 
account of our knowledge of the development of these forms up 
to the date of its issue (1879). That there is a development with- 
out fertilization is admitted on all sides, but the question as to 
whether the vedig develop from true germ cells is still a peint of 
dispute. Leuckart (1882) and Schwarz (1886) consider this as a 
true case of pedogenesis, the internally developing redie being 
looked on as arising viviparously from cells of the germinal epi- 
thelium. On the other hand, Wagener (1857) and Biehringer 
(1885) maintain that they arise from cells of the body wall and are 
therefore not produced sexually but by budding. Korschelt and 
Heider, in their Zext-Book of Embryology (1890), do not consider 
this difference of great significance. ‘‘*This difference does not 
seem to us to be important, for we have already seen that the 
parietal cells and the germ cells are embryologically of the same 
origin. In a portion of the cells of the body wall even, a differen- 
tiation into separate histological elements appears not to have 
taken place, and for this reason they may continue to develop in 
the same way as the real germ cells. In harmony with this view is” 
the statement of Thomas,* who derives the rediz from both the 
germ cells and the cells of the body wall; if the supply of the 
former were exhausted, then the latter might take their place.”’ 
1]J. Bd., p. 488 and following pages. 
? Page 183, Vol. I, English Translation. 
5’ Thomas (1883). 
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