506 Development of Mouth and Gills in Bdellostoma 
endodermal outpushing has become more pronounced, reaching to the 
ectoderm. At this stage an ectodermal nerve placode appears just dorsal 
to the gill. Following this, the remaining seven or eight gills are still 
spread out on marginal lappets. The present figure was taken from 
the same embryo which furnished Fig. 16 of the hyomandibular cleft 
and Fig. 20 of the first thyroidean gill. 
Fig. 25, from an embryo 15 mm. in length, shows the first gill still 
at the base of the lateral gill lappets with the endodermal diverticula 
much greater in extent and the ectodermal thickening but little changed. 
The extreme anterior parts of the second and third gills are indicated 
by the two indentations in the endoderm g2 g3. ‘These gills are now be- 
ginning to be drawn in from the lateral lappets as the closure of the 
fore-gut progresses. And at this stage the hyomandibular and first 
thyroidean gills have already been shown in Figs. 17 and 21 respectively. 
The fore-gut continues to close by the drawing together of the sides of 
the gill lappets, and thus the gills are brought in along the sides of the 
neck in the manner which Dean has already described. The shifting does 
not begin until this process is almost completed. 
Pouching of the Gills——Before the gills begin to shift backward the 
endodermal diverticula are coming to form chamber-like cavities a short 
distance from their proximal ends. Fig. 27 shows a section through a 
gill at this stage the swollen cavity, gp, is seen near the pharynx, g, the 
connection of the gill tube with the pharynx is anterior to this section, 
the gills all having a trend from the gut caudally to the ectoderm as 
will be seen by referring to the diagram Fig. 35. This swelling of the 
gill tube is the earliest step in the formation of the complex gill pouch 
of the adult. The progressive changes in the endodermal gill chamber 
may be seen by studying the sections shown in Figs. 27 to 32. In Fig. 
28 the wall of the chamber is beginning to be folded and in Fig. 29 the 
pouch is already assuming a rather complexly folded condition. In 
Fig. 30 the pouch is seen to communicate with the pharynx, the outer 
tube, 7. ¢., leading from the pouch to the ectoderm, appearing only in a 
more posterior section. In this section the ectoderm is seen to form 
a slight fold, ep, margining the end of the endodermal tube. At this 
time the pouch begins to grow rapidly, as will be noticed by comparing 
the several figures all drawn to the same scale. The gills at the stage 
shown in Fig. 30 have shifted far back, almost as far as in the fully 
developed condition. In Fig. 31 is seen a section of a somewhat further 
developed pouch from the mid gill region of the same embryo of which 
Fig. 30 represents the most anterior pouch. The pouch of Fig. 31 
