Development of the Infusoria, ' 247 



daily of a lighter colour, particularly at the margins ; others 

 throughout of a lighter colour. No trace of motion was obser- 

 vable in the dark ones, while in the latter the embryo often 

 made vigorous movements, and the ciliae were seen distinctly 

 vibrating in the interior of the egg. The former were therefore 

 evidently the least developed ova. We may therefore perhaps 

 form the following scale of their development : 



1. The dark opaque ova I reckon the most unripe; they 

 vary in size, and are more or less elongated. When pressed 

 between plates of mica, the granular homogeneous-looking sub- 

 stance in the interior is discharged (the yolk), and the empty 

 transparent envelope remains (the chorion). I could never ob- 

 serve any thing more than this granular substance, and its con- 

 taining membrane. I could never detect a second coat, the 

 amnion of Ehrenberg. The chorion had a remarkable ap- 

 pearance, being beset with short, fine, dense hairs, resembling 

 the closest fur. This hairy covering is very apt to escape no- 

 tice. I did not observe it at first ; but, after having once seen 

 it, I found it present in every case. This villous covering, 

 which reminds us so closely of the chorion of the higher ani- 

 mals, is most distinct in unbroken opaque ova. It is, however, 

 visible, though much less distinctly, when their contents are dis- 

 charged. This structure is the more interesting, as Meyer in- 

 forms us (Okens Isis, 1828, p. 1225), that the envelope of the 

 egg (chorion) of the Alcyonella is beset with ciliae. Grant 

 found it on the ova of the sponge, and Carus views it as the 

 nucleus of the respiratory vessels or gills of the chorion. 



2. An egg has sometimes an opaque nucleus, and border, 

 when the rest of the granular mass is transparent. This also 

 was beset with hairs. I am not sure whether this is the second 

 stage of the development or a dead ovum. 



3. The ova of this stage are much more transparent, filled 

 with a brownish-yellow, granular substance (yolk) ; some spots 

 are darker, and resemble turbid fiocci in the midst of the ovum. 

 Some organization is discernible in the interior, but there is no 

 distinct separation of organs. Some resemblance seems to exist 

 to the embryo of the common leech, where the first change per- 

 ceptible in the globular ovum is the formation of cells and ve- 

 sicles. 



