the Ciliate Infusoria. By G. J. AUman. 
179 
portion of an Amoeba. The nucleus is imbedded in the inner side 
of the cortical layer, which is itself differentiated into certain 
secondary layers. He describes the deeper part of the cortical 
layer as exhibiting a rotation of its granules independent of the 
rotation which occurs in the central parenchyma, and moving in a 
direction opposite to that of the latter. Everts's account of the 
structure of Vorticella is thus in accordance with the conception of 
it as a cell with a parietal nucleus; a cell, however, in which 
differentiation is carried very far without the essential character of 
a simple cell being thereby lost. 
Everts regards the external wall as corresponding with the 
ectoderm, and the internal softer body-substance with the endoderm 
of higher animals. If by this the author meant to indicate a 
homological identity between the structures thus compared, it is 
plain that he would have taken an entirely mistaken view based on 
a misconception of the essential nature of an ectoderm and 
endoderm. These membranes are essentially multicellular, and 
are always results of the segmentation of the vitellus in a true 
ovum. They can therefore never be attributed to a unicellular 
animal, in which no true segmentation process ever takes place. In 
his rejoinder, however, to an elaborate criticism of his memoir by 
Greeff, he explains that he intended to compare the two layers of 
the Infusorium body analogically, not morphologically, with an 
ectoderm and endoderm. 
The same author has further made some interesting observa- 
tions on the development of Yorticella. He has noticed that repro- 
duction is here ushered in by a longitudinal cleavage, in which 
after division of the nucleus the body of the Yorticella becomes 
cleft into two halves, still seated on the common stalk. Each of 
these develops near its posterior end a wreath of vibratile cilia, 
while the peristome and the cilia disk over the mouth are entirely 
withdrawn, and then breaks loose from its stem and swims freely 
away. These free-swimming Yorticellse now encyst themselves, the 
cilia disappear, and the contents of the encysted animal acquire a 
uniform clearness with the exception of the nucleus, which persists 
unchanged. In the next place the nucleus breaks up into eight or 
nine pieces, and then the wall of the cyst becomes ruptured and 
gives exit to these fragments, which now appear as spontaneously 
moving spherules. These increase in size, develop on one end a 
cilia wreath, within which a mouth makes its appearance, and the 
free-swimming nucleus-fragment becomes gradually changed into a 
form which entirely agrees with the Trichodina grandinella of 
Ehrenberg. 
These Trichodinse now multiply by fission, first developing a 
posterior wreath of cilia, and then dividing transversely between 
the anterior and posterior wreaths. After this each fixes itself by 
VOL. XIV. o 
