1885.] PROCEEDINGS OF UNITED STATES NATIONAL MUSEUM. 151 



The posterior branchial arches support clusters of teeth, which, in 

 the sections studied by the writer, presented the forms represented in 

 Figs. 26 and 27. A saccular organ, <?/, was developed from the deeper 

 layer of the epidermis, in which the conical crown of the tooth, d, is 

 first formed, as shown in Fig. 26, but with the progress of development 

 the perichondrium pch, Fig. 27, which invests the cartilaginous bran- 

 chial bar Br, forms a basal cushion, s, s, s, for each tooth, the conical 

 crowns d, d, d being evidently articuhited to the summits of these 

 cushions, which give rise to the so-called cementum plates. It would 

 thus appear that the crowns of the ])haryngeal teeth, like the dermal 

 denticles of certain forms, arose separately from the basal parts, which it 

 is very clear develop later than the crowns in this instance, in intimate 

 relation to the perichondrium investing the branchial bar, and there- 

 fore have a deeper origin than the crowns. The saccular organ, df, is 

 developed from a more or less nearly solid diverticulum of the deep 

 layer of the epidermis, whibh, in some cases, finally loses its connection 

 with the latter, and becomes hollowed out interiorly into a cap-like organ 

 in which the germ of the crown appears. 



The branchial filaments in the advanced embryo shown in Fig. 3 

 already have cartilaginous axes, c, as shown in Fig. 17 ; the base of the 

 axis of the filament being furcate, and resting astride of the branchial 

 vein vh, and not quite in contact with the branchial arch bh itself. 

 These filaments are already pinnate and vascular. 



The vertebral column. — The anterior part of this organ has already 

 been described. The vertebrae are best differentiated anteriorly ; in 

 the tail the membrane in which the ossification of the centra occurs is 

 apparent but not so strongly marked as anteriorly. The chorda bends 

 upward but slightly at its hinder end, so that the urostyle is finally very 

 short, as shown in Fig. 18. 



The ribs are developed in cartilage as a single line of longitudinally 

 compressed cells, as shown in Figs. 19 and 23 at rb. In the anterior 

 thoracic region the heads of the ribs are elevated above the level of the 

 chorda and are embedded in the fibrocartilaginous rudiments sic of the 

 neural arches, as shown in Fig. 23; posteriorly the ribs have their 

 heads lowered more and more and assume a more ventral and intimate 

 relation to the skeletogeuous sheath of the chorda. 



The fin-rays of the caudal fin in the most advanced foetuses are al- 

 ready pretty well developed in membrane, beneath the epidermis ; their 

 relations to the mesoblast me internally and externally to the malpighiau; 

 layer si and epilthelium ep being shown in the cross-section repre- 

 sented in part in Fig. 24, and in Fig. 9 at t. 



Visceral anatomy. — The way in which the foetus is coiled up in the 

 follicle is shown in Fig. 4, in which the process of the yelk-bag 2>y, which 

 extends upward over the opercles and to the top of the head, is also 

 shown. In Fig. 9 a section through a follicle with the embryo in place 

 and cut through at three different points is shown. This section also 

 shows the open iollicalar pore my with a thickened margin and the fol- 



