520 DR MASTERMAN ON THE FURTHER ANATOMY AND 



A nearly median longitudinal section through the bud (fig. 26) is illustrated 

 in fig. 60. The buccal shield attains at this stage its greatest proportionate size, 

 and within it we may notice some important differentiations. The sub-neural gland 

 (sn.gl.) has moved backwards to reach the dorsal wall, and has now come at its distal 

 end into close relationship with the sub-neural sinus (sn.s.). The postero-ventral 

 wall of the latter has become slightly pushed in (at least in the preserved specimen) 

 and the mesodermic wall of the invaginated part becomes thickened by a grouping of 

 mesodermic nuclei and protoplasmic strands. Certain of these cells protrude into the 

 cavity of the sub-neural sinus in a symmetrical radiate manner, the first traces of 

 which may be sometimes recognised in the earlier buds with two plumes (pr.s.). 

 The anus now opens as a wide aperture to the exterior, in the same dorsal position 

 as in the adult. 



The stage with three pairs of plumes develops from the preceding in quite a 

 regular manner by the formation of a third pair, at first in the same straight line as 

 the other two, and in the ordinary course the bud becomes detached from its parent 

 at this stage. This is effected by a simple construction of the end of the pedicle where 

 it is fused with the parental pedicle. The connecting tissue becomes narrower and 

 narrower, the ectoderm presses in on all sides upon the ccelomic channel, whilst at 

 the same time sections through a late stage of the process indicate a certain amount 

 of histolysis especially in the longitudinal muscles. Specimens can be obtained in 

 which the junction is so narrow that a gentle shake serves to detach the bud from 

 its attachment, and sections of the bud show that the ccelome is closed and that the 

 ectoderm although thin is continuous over the tip of the pedicle. In these cases one 

 might fairly assume that the bud remains in contact with the parent only by virtue 

 of the adhesive power of the pedicle. The free buds at this stage are roughly about 

 £ the bulk of the adult. No reference up till now has been made to the size of the 

 buds which have been described. The reason for this omission is that the size of 

 the buds corresponding to any particular degree of differentiation varies within widi 

 limits, so that no fast rule can be stated. 



The size of the early buds in fig. 19 can be gauged by comparison with the peclick 

 to which they are attached, and the same applies to the four-plumed stage in fig. 25. 



On the other hand the rest of the figures except fig. 28 are drawn to nearly the same 

 scale as fig. 25. Fig. 27 illustrates the appearance of the bud soon after detachment. In 

 this instance the buccal shield and post-oral lamella have been removed to show the 

 arrangement of the plumes, which will be alluded to later. Fig. 28 gives the entire 

 bud in side view, at about the same stage. The buccal shield is still of enormous size 

 and. partially over-laps the short contracted pedicle. Sections through such a stage 

 show that except for the plumes and gonads the bud may be regarded as a small- 

 sized adult. 



Fig. 61 shows that the pharyngeal clefts are now open to the exterior, and that 

 their walls are assuming a chordoid character, and fig. 62 indicates very clearly the 



