1902.] NATURAL SCIENCES OF PHILADELPHIA. 101 



tlie gland were straigliteued out the duct would then be consider- 

 ably nearer the anterior end. By comparing fig. 6c with this dia- 

 gram, it will be noticed that the duct is now a longitudinal slit 

 instead of a nearly circular duct, the lateral growth of the duct 

 apparently not having kept pace with the longitudinal. Fig. 6k 

 represents the actual relative longitudinal but not vertical dimen- 

 sions of the duct. The actual increase in size of the gland may be 

 appreciated by noting that, though all the sections were drawn with 

 a camera, those represented in figs. 56-/ are enlarged 240 diame- 

 ters, while those in figs. 6a-i are magnified only 90 times. 



A section through the anterior end of the gland, passing through 

 the point marked a, fig. Qk, is represented in fig. 6a. The chief 

 difference, besides that of size, betAveen this section and the corre- 

 sponding one of the preceding stage (fig. 5/) is in the complete 

 development of the cell-gi'oups (gr'.), which in the preceding stage 

 were quite small and imdeveloped. We have now in each side of 

 the gland, which is exactly bilaterally symmetrical, four groups of 

 gland cells, the groups being arranged in pairs, one pair on each 

 side lying near the middle line, the other being nearer the side 

 (figs. 6e and k, m.c.g. and Leg.). This arrangement of the cell- 

 groups into pairs is distinct from end to end of the gland, and for 

 convenience they will be spoken of as the ' ' median ' ' and ' ' lat- 

 eral " pairs of cell-groups. As has been said, the gi-oups gr'. 

 never reach the size of the first developed groups gr. 



The ciliated cells have changed somewhat since the preceding 

 stage. They have become relatively, as well as actually, more 

 elongated at certain parts of the ciliated layer, while at other places 

 they are still sliort and nearly filled with their large oval nuclei. 

 The nuclei of the longer cells are small and circular in outline, and 

 at certain places seem to be more or less regularly arranged near one 

 end or other of the cells, while at other places there is no apparent 

 regularity in their arrangement. In fig. 6a, the short cells with 

 large nuclei are seen on either side of the slit-like openings (o. ) 

 into the groups of gland cells. The ciliated grooves {v. e.g.), at 

 the point where this section is cut, arc still some distance apart on 

 the fioor of the pharynx, and are not noticeably changed from their 

 condition in the preceding stage. They are separated by one large 

 and two small folds in the fioor of the pharynx, which seems, at 

 this point, to be very irregular in outline, probably caused partly 



