282 MENSCH. [Vol. XVI. 



parent stock. The first indication of these eyes is marked by 

 the appearance of minute pigment spots in a stage intermediate 

 between this and stolon i, at a time when the lateral and dorsal 

 median tentacles have become about one-half as large as repre- 

 sented in stolon 2. 



A lateral view of the developing head of this stolon is repre- 

 sented in PI. XIII, Fig. 8. In this view it will be seen that 

 the zone of new growth extends forward considerably more 

 than in the preceding stolon, and also, that instead of extending 

 down laterally as far only as the parapodia, it now extends to 

 the ventral median line. The whole structure at this stage 

 assumes the appearance of a separate segment, being narrower 

 above and narrowing down laterally until on the ventral surface 

 it forms a very narrow strip of tissues, the outlines of which are 

 lost as it approaches the median line. On the lateral surface 

 of this new growth, a little dorsal to the insertion of the dorsal 

 cirri, a small papilla {d.t}) has made its appearance. This bud 

 represents the rudimentary dorsal tentacular cirrus, and first 

 appears at about this stage of development, being somewhat 

 later than the dorsal median tentacles, which may be found in 

 stolons of a stage of development intermediate between this and 

 stolon I. In a stolon of this age, then, there are present : (i) a 

 pair of anterior lateral tentacles ; (2) the dorsal median tentacle ; 

 and (3) the pair of dorsal tentacular cirri, all having appeared 

 successively in the order enumerated. Since the tentacular 

 cirri form a part of the buccal segment, it may be assumed 

 that already in this stage this new tissue is being differentiated 

 into the buccal and head segments, even though there are no 

 external indications of this division. 



Between the anal segment of the preceding stolon and the 

 head parts of stolon 2 (PI. XIII, Fig. 8) there exists a narrow 

 structure which belongs, properly speaking, to neither of these 

 segments, and which I shall designate as the region of sepam- 

 tion (r.s.). This region forms the connection between the two 

 stolons, and in earlier stages seems to belong to and constitute 

 an undifferentiated part of the anal segment. At a stage repre- 

 sented by these two stolons, however, it seems, externally at 

 least, to be entirely distinct from the anal segment, and presents 



