32 LECTURE II. 



Roeselii e. g., presents to the spirally disposed elongated head of the 

 spermatozoon in the Torpedo, Pelobates, and the Passerine birds. 



Besides the more frequent and commonly observed mode of propa- 

 gation by spontaneous fission, the Polygastria generate in the more 

 normal and perfect way. This, indeed, has been rarely seen, and has 

 been denied by some.* No doubt the term 'ova' has been applied to 

 many of the minute granules and nucleated cells in the soft paren- 

 chyme of the Polygastria, without that evidence which is requisite 

 to produce conviction of the accuracy of such determination. The 

 green-coloured nucleated cells, for example, which circulate in 

 Loxodes, like the chlorophyll-particles in the leaf-cells of Valisneria, 

 are more probably concerned in fixing cax"bon and eliminating 

 oxygen, like their answerable parts in plants. But the essential 

 part of the ovum, e. g. the germ-cell, must exist to set on foot those 

 processes of development which lead to the formation of the embryo 

 in the viviparous species of Polygastria. And I may here cite the 

 remark of a distinguished chemist in support of the partial accuracy 

 at least of Ehrenberg's ascription of ova to the Infusoria. In the 

 simplest animals in which the ova or ovarium can be distinguished 

 with certainty, it is the only organ in which oil or fat is accumulated. 

 The yellowish masses which Ehrenberg discovered, by the aid of a 

 high magnifying power, on each side of the siliceous carapace in the 

 gelatinous envelope of the Frustulia salina, when chemically tested, 

 yield abundance of fat : tl.ey disappear, e.g. when treated with aether, 

 and the latter then contains a brownish fat in solution. This result 

 of Schmidt's minute analysis leads that accurate observer to regard 

 Ehrenberg's determination of those yellowish masses, as ovaria, to be 

 well founded."!" The formation of locomotive germs in, and their 

 escape from, the interior of a Polygastrian, appears to have been 

 seen by Dr. Arlidge % in the Trichodina pediculus, in which he de- 

 scribes the phenomena as a kind of internal gemmation. The best 

 description of the viviparous generation of a Polygastrian is that given 

 by Focke § and Cohn || in Loxodes Bursaria. In this species, at the 

 latter end of autumn and in winter, there may be seen within the 

 body one or more large globules, which, when from six to eight in 

 number, present, by mutual pressure, a parenchymatous structure 

 {Jig. 19.). They are of different sizes, from -y^'" to -xW" ^^ ^^^' 

 meter, well-defined, almost colourless, filled with fine granules and 

 one or more hyaline nuclei (Ji) ; and they are inclosed by two 

 contractile cysts, defining the individual life in each. These germs 



* XXIV. p. 23. t I^- P- 36. % XXXI. 



§ XXXIL II XVIII. 



