600 Bashford Dean Memorial Volume 



is necessary. Note that in both heads, the isthmus grows progressively narrower from 

 back to front. Short external filaments are found on each side of each arch. 



Scammon's stage of Squalus nearest to this 32'mm. Chlamydoselachus is one of 

 28'mm. portrayed in fulMength lateral, dorsal and ventral aspects (his Fig. 30a, b and c, 

 pi. III). Since there is no fulMength drawing of the 32'mm. Chlamydoselachus, com' 

 parisons are difficult. But comparisons of heads only show that this 28'mm. Squalus is 

 much farther developed than the 32'nmi. Chlamydoselachus. This is particularly true in 

 the gill'tegion. The spiracular cleft of the dogfish is finished, and from it and the other 

 clefts a profusion of long external gill'filaments protrude. 



Dean's Embryo 34 mm. in Length 



The "List of Figures" calls for "Embr. of 34 mm. Entire: head 2 other positions". 

 And fortunately there are of this embryo a fulMength drawing in lateral aspect (223 mm. 

 long in the original) shown in Figure 23, plate II, and also dorsal and ventral figures of the 

 head. Comparisons will be made of the head of this specimen in three aspects with 

 the drawings of the head of the 32'mm. embryo. In addition comparison of the figure 

 of the 34'mm. specimen in full lateral view will be made with the similar figure of the 

 20'mm. embryo. 



Head in Dorsal View. — As may be seen in Figure 22, plate II, the head of this 

 embryo contrasts strongly with that of the 32'mm. specimen. It is bluntly rounded and 

 the eyes are less prominent. Let the reader contrast the markings on the head over the 

 brain in the two figures. I do not understand them unless they are due to shrinkage of 

 the soft tissues on the embryonic skull. Let the observer particularly note that the open' 

 ings of the endolymphatic ducts are no longer visible. The spiracular openings are 

 smaller. The gill'covers are less widely spread and in this aspect no filaments are seen in 

 them. The pectorals are about as they were in the 32'mm. embryo. 



Lateral Aspect, Head Only. — The head in this view (Figure 23, plate II) should be 

 compared with the lateral view of the head of the 32'mm embryo (Figure 20, plate II). 

 The head of the 34'mm. embryo is more rounded. The eye still shows the choroid 

 fissure. The greatest progress however is to be noted in the mouth and spiracle. The 

 mouth begins to look somewhat like that of a shark. Most noticeable is the fact that the 

 cheek'piece so conspicuous in the 32'mm. embryo (Figure 20, plate II) has here grown 

 fast to the hyoid arch. The external opening of the spiracle is much smaller and is in line 

 with the medial borders of the gill'slits. The first gill-sHt is very large, and its arch and 

 all the other arches show backward folds where they join the body above and the 

 isthmus below. 



Head in Ventral View. — In this aspect (Figure 24, plate II) it is noticeable that the 

 nasal pits are smaller. The fossa between the cartilages of the upper jaw is much reduced, 

 the mouth itself is less widely open than in the 32'mm. specimen (Figure 21, plate II), 

 and is more adult in appearance. The reduction in width of the isthmus from the region 

 of the sixth flap forward to the first is very noticeable. The distended first gill'Covers 



