180 bulletin: museum of comparative zoology. 



activity certainly exists. It may well be that excretion in the embryo 

 is less abundant than in the predatory larva, because of the less active 

 metabolism and the more perfect food supply of the yolk. The presence 

 or absence of excretion products in the organ during embryonic life 

 might be demonstrated on fresh material by micro-chemical tests, and 

 the question thus definitely settled ; but my material is not so preserved 

 as to permit the application of such tests. 



After the formation of the ectodermic ingrowth, as shown in Figures 

 26 and 27 (Plate 3), the further embryonic development consists, then, 

 in four chief phases : — (a.) The line representing its lumen extends to 

 the outside world and so establishes a potential duct. This is a rapid 

 process and is completed shortly after the stage shown in Figure 28 

 (Plate 3). The condition previously noted at the deep ind of the ecto- 

 dermic ingrowth — several rows of cells — progresses and finally reaches 

 the exterior near the point where the ectodermic ingrowth first appeared 

 (Fig. 25). This potential duct is open in many cases (Fig. 29). The 

 opening of the duct is on the anterior side of the appendage, somewhat 

 toward its ventral face. The ectodermic duct and sac extend from this 

 external orifice dorsad and proximad, covering the distal and dorsal faces 

 of the endsac, but not until a later stage does it pass ventral to the end- 

 sac, nor ever much beyond its proximal border. 



(b.) The ectodermic sac becomes extended and complicated through- 

 out embryonic life by the formation of evaginations, M^hich begin to ap- 

 pear as soon as the lumen of the duct has been extended to the external 

 world. These outgrowths are flattened, probably by mechanical pressure. 

 There are three regions at which principally these evaginations occur. 

 One of these is proximal, i. e. at the deep end of the ectodermic ingrowth. 

 Here the outgrowth soon becomes forked, there being a ventral portion 

 nearer the endsac and a proximal portion more nearly in line with the 

 axis of the appendage. Figure 30 (Plate 3) shows these forks, but the 

 proximal one is seen only in cross section. It extends at right angles to 

 the main axis of the appendage and to the plane of the section, approx- 

 imately parallel to the frontal plane of the embryo. The notch separat- 

 ing these forks never becomes very deep, but it is recognizable from the 

 second month of embryonic life until the late larval stages. On the 

 dorsal face of the ectodermic sac in the region of its sharpest curvature 

 is a second area of evagination. This outgrowth, which is dorsad, does 

 not exist at the stage shown in Figure 30, but it appears slightly later. 

 It has a considerable horizontal extent (Figure 31, Plate 3), being all 

 that part of the ectodermic sac which later lies distal to the duct. The 



