The Embryology of Chlamydoselachus 607 



■■■■'■. An Embryo OF 54 Millimeters 



In Dean's "List", the next call is for "Embryo of mm. 55. Entire — draw dorsal and 

 ventral views [of head]". These drawings I find. But, in the plates of newer drawings of 

 later origin (as noted above), I find three full-length drawings — in dorsal, lateral and 

 ventral aspects — of an embryo labelled "54 mm." in Dean's writing. The full-length 

 figures of the 54-mm. specimen will now be contrasted with those of the 46-mm. embryo. 

 The figures of the 55-mm. fish (belonging to the older set of drawings) will be studied 

 next. Each of the original drawings of the 54-mm. embryo measures 257 rnm. — i. e. 

 is multiplied by 4.7- 



Dorsal Aspect. — When comparison of this drawing (Figure 32, plate III) is made 

 with a similar one (Figure 28, plate III) of the 46-mm. specimen, the head and trunk are 

 found to be notably larger. The fish is decidedly like an elongate tadpole. The latero- 

 sensory canal system on the head is clearly seen. The first gill-covers are not so widely 

 spread. From all the gill-slits profuse elongate filaments contrast with the shorter 

 ones of the younger embryo. Then, too, from the spiracle protrude more and longer 

 filaments. The pectoral and pelvic fins show decided growth, but in this aspect one 

 cannot say about the dorsal fin and the upper lobe of the caudal. The body between 

 pectorals and pelvics is relatively shorter than in the 46-mm. embryo. 



Lateral View. — Marked contrasts may be drawn between the 54- and the 46-mm. 

 embryos seen in lateral aspect (Figure 33, plate III, and Figure 29 on the same plate III). 

 The 46-mm. fish is very sway-backed, the 54-mm specimen has a marked concavity in 

 the neck region but behind this it is moderately hump-backed and has something of the 

 look of the adult fish. The head is rounder and better developed than in any previous 

 stage. The pits of the latero-sensory canal system are well developed over head and 

 first gill-cover. The lower jaw has become greatly elongated and faintly recalls that 

 of the adult. The almost vertical hinder edge of the first gill-cover contrasts marked- 

 ly with the open U-shaped structure of the 46-mm. specimen — which is probably 

 anomalous. The external gill-filaments are well developed. Some are found in the 

 spiracle, which is much higher up on the side of the head than that of the 46-mm. fish. 

 Both paired and unpaired fins of the 54-mm. fish are somewhat better developed than 

 those in the younger one. The tail of the present fishlet is as much bent up as that of the 

 46-mm. specimen is bent down. Other than this, the tail regions are much alike. 



Ventral Aspect. — Considerable contrast is to be noted when the two fish are com- 

 pared in ventral view (the 54-mm. in Figure 34, plate III, and the 46-mm. fish in Figure 

 30 of the same plate. The head of the 54'mm. specimen is much broader as was noted in 

 dorsal aspect. The eyes are larger, the openings of the olfactory slits smaller. The pits 

 of the latero-sensory canal system show clearly. The mouth shows marked develop- 

 ment — it resembles that of the adult but is still ventral in position. The first gill-covers 

 form a wide cloak covering the isthmus. The gill-arches — standing out fairly at right 

 angles in the younger specimen — are here bent somewhat backward. Every gill-slit is 

 crowded with external filaments which reach their maximum development here. Trunk 



