LIXNEAN SOCIETT OF LOXDON. XIX 



view of testing the statements of Greeff, he made on Vorticella 

 nebulifera. GreeflP, as we have seen, attributed to the Yorticellce a 

 true ccelenterate structure ; and Everts, by his own investigations, 

 has convinced himself of the untenableness of this view, and has been 

 led to regard the Vorticellce as strictly unicellular. 



He recognizes the distinction between the cortical layer, which 

 forms not only the periphery of the body, but the whole of the stalk 

 on which this is supported, and the central mass in which the nutri- 

 ment is deposited, collected into pellets and digested. The nucleus 

 is imbedded in the inner side of the cortical layer, which is itself 

 differentiated into certain secondary layers. Everts's account of the 

 structure of Vorticella is thus entirely in accordance with the con- 

 ception of it as a cell with a parietal nucleus — a cell, however, in 

 which differentiation is carried very far without the essential cha- 

 racter of a simple cell being thereby lost. 



Everts regards the external wall as corresponding with the ecto- 

 derm, and the internal softer body-substance with the endoderm of 

 higher animals. If by this the author meant to indicate a ho- 

 mological 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 au ectoderm and endo- 

 derm. 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, how- 

 ever, 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, v.ith an ectoderm and endoderm. 



The same author has further made some interesting observations 

 on the development of Vorticella. He has noticed that reproduction 

 is here ushered in by a longitudinal cleavage, in which, after divi- 

 sion of the nucleus, the body of the Vorticella 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 and the cilia-disk over the mouth are entirely withdrawn, and then 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 Vorticellce 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 frag- 



c2 



