594 Bashford Dean Ivlemorial Volume 



This same specimen was stained, cleared and drawn somewhat enlarged (to l6l 

 mm. — X 14), apparently in order that the neuromeres and myomeres might be studied and 

 counted. In this Figure 16, plate II, there is plainly seen between fore-and midbrain 

 a small rounded body which I take to be the rudiment of the epiphysis. The neuromeres 

 are divided into two sets of three each by a very short neuromere (?) which I do not under- 

 stand. The auditory vesicle is prominent, standing over the bar between the second and 

 third gill-clefts. From this vesicle to the end of the tail are 102 myomeres. This was 

 noted in pencil on the original drawing. Plainly visible are six gill-clefts, the seventh 

 being very faint. The heart is prominent — as is a large blood vessel, the vitelline artery, 

 branching off from the dorsal aorta. The cloacal eminence is very prominent, and in 

 front of it is a thickening which is presumably the rudiment of the pelvic fin. The fine 

 line bounding the entire figure represents the superficial ectoderm. 



An Embryo 15.5 mm. in Length 



The next drawing called for in Dean's ''Material & List of Figures" portrays an 

 embryo of this size in lateral aspect (Figure 17, plate II). This drawing is labelled "Emb. 

 B 15.5 mm." on the drawing. This embryo agrees in length and in development very 

 closely with Scammon's stage No. 26 (his pi. II), a 15-mm. Squalus. Dean's figure is 

 marked "X10+" but in the original drawing it measures 177 rnm. From the head, the 

 back line slopes down to a point about over the yolk-stalk junction. Thence it runs back- 

 ward almost straight to a point over the cloaca, from which region the tail bends down 

 sharply. The forebrain together with the olfactory rudiment is prominent and is slightly 

 upturned. The midbrain is large and bulges forward strongly. Thus the profile of the 

 head of this embryo has a striking resemblance to that of a bulldog — though the parts do 

 not correspond. The epiphysis is indicated by a slight swelling above the forebrain and 

 in front of the eye. The optic cup and lens show some enlargement. The mouth still 

 gapes widely. 



The prominent bulge in front of the yolk is due to the presence of the heart. The 

 pectoral fin is larger than that of the 11.5-mm. embryo. On the ventral surface of the 

 body is a ridge, probably an evidence of the beginning of the gut. The cloacal swelling 

 is very marked. In front of this is a thickening, presumably the anlage of the pelvic fin. 

 Above the cloacal eminence, the straight dorsum slants downward as the tail. This ends 

 in a curious upward hook Hke that found on the caudal extremity of a Boston terrier 

 whose tail has been bobbed and the point bent upward. 



In this 15.5-mm. Chlamydoselachus (Figure 17, plate II) the gill-arch region is very 

 prominent. Some of the gill-arches appear crumpled. This crumpling is, I judge, an 

 artifact due to shrinkage. In the dorsal portion of the branchial region, there appear 

 three ridges that resemble incomplete gill-arches. Of these, the two anterior ones are 

 probably not gill-arches but parts of the cranium, while the third is a portion of the first 

 visceral arch. One notable difference between the 15.5-mm. embryonic Chlamydoseh 

 achus (Figure 15, plate II) and the 15-mm. embryonic Squalus, shown in Scammon's 

 Fig. 26, pi. II, is that the little Squalus has at least one gill-filament projecting from each 

 first, second and third sUt, whereas these are entirely lacking in our embryo. 



