172 Dr. R. Leuckart on Asexual Reproduction. 
to the alternation of generations in the Aphides, but distinguished 
from the usual forms of this mode of reproduction with larviform 
nurses by the circumstance that the sexual individuals do not 
from the first possess their ultimate form, but only acquire this 
by a supplementary metamorphosis. The alternation of genera- 
tions in the Distoma, however, presents us with an approxima- 
tion to these conditions, inasmuch as in this case also the newly 
born sexual animal (Cer caria) represents a creature which only 
becomes matured into the definitive form after undergoing cer- 
tain transformations. 
Addition. 
Since the preceding was written (in the middle of January 1865) 
the viviparous Cecidomyide larvee have been uninterruptedly ob- 
served by us. The larvee conveyed into a warm room thrive ad- 
mirably, grow, and produce germs, the development of which 
proceeds in a normal manner, whilst in the open air, as above 
described, they are sooner or later destroyed by fatty degeneration. 
In our climate, therefore, the propagation of the larvee must be 
usually interrupted by the winter, to recommence as soon as the 
warm weather sets in. 
The abundant material for observation (even of the later stages 
of development) now before us has gradually given us a tolerably 
completeinsight intotheembryonic development of the pseudovum, 
as will appear from the forthcoming memoir by M. Mecanikoff, 
to whom I have handed over the material for further investigation. 
Of this I will only anticipate one point—namely, that the large 
balls which lie upen the embryonal yelk (with a blastoderm) 
described by me above as the remains of the formative cells of 
the vitellus, have proved to be so-called polar cells, which belong 
to the hinder end of the germ-chamber, and, according to the 
interesting discovery of M. Meczmkoff, which I can fully con- 
firm, finally pass into the germ-stock of the young larva. The 
formative cells of the vitellas have already disappeared in the 
stages with a developed blastoderm (which alone were previously 
observed by me), but are still present when the formation of the 
latter commences. This error might have been avoided, if the 
first processes of embryonic development had come earlier under 
my observation. However, this has no influence on the concep- 
tion of the conditions in general; in this respect I could only 
now repeat, word for word, what I wrote at the time. 
For the characterization of the larva, I may also state that it 
has only two stigmata in the earlier phases of its existence, but 
afterwards acquires a greater number (five pairs). In this later 
condition, moreover, the granulation of the ventral plates is 
somewhat different, and the first segment of the body is coalescent 
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