92 ARTHUR BOLLES LEE. 



larly ovoid, or even almost globular (fig. 3). In Salps of from 

 5 to 7 mm. in length it is more elongated, and presents a 

 contour of an elegant double curvature, the basal portion con- 

 taining the vestibule being much more drawn out than in earlier 

 stages (see fig. 4). It is composed of clear, flattened, squa- 

 mous cells, thicker at their proximal margins than at their 

 distal margins. Their nuclei, loose-textured like those of the 

 epithelium of the stem, occupy the proximal region of the 

 cells. 



There are two layers of these cells [ext. ca. and int. ca. in 

 figs. 3 and 4). By reference to fig. 3 it will be seen that doubt 

 is permissible as to the proper way of describing the position 

 of these two layers. The cells marked int. ca. appear to be 

 internal with regard to the cells marked ext. ca. ; but they 

 might also fittingly be described as ''upper" or "distal" cells. 

 And seeing that the calyx is evidently only an expanded portion 

 of the tube of the stem, which is lined by only a single layer 

 of epithelium cells, this may appear the more fitting view. I 

 incline, however, to the former view. The impression left on 

 mv mind by a most careful study of this object is that the 

 present arrangement is the result of the invagination of the tip 

 of a primitive hollow outgrowth of the inner tunica, the inva- 

 ginated cells forming an inner layer to the cup thus produced, 

 and the sense-cells being differentiated from the cells at the 

 fundus of the cup, or, if previously formed at the tip of the 

 outgrowth, being iuvaginated with the rest. It should be added 

 that I have many times distinctly observed the nuclei of the 

 outer cells to be placed on their outer wall, whilst the nuclei 

 of the inner cells were placed on their inner wall, which is the 

 place they would occupy if the latter are only an iuvaginated 

 state of the former. 



Attempts to elucidate this point by direct observation of the 

 genesis of the organ have failed, as it was found that, owing to 

 the very precocious development of the organ, an arduous study 

 of serial sections of the developing stolon would be necessary, 

 and the case does not seem to warrant the expenditure of so 

 much labour. 



