744 Bashford Dean Memorial Volume 



Figure 28, plate II, represents a surface view of an embryo with at least 50 complete 

 somites. It is drawn at a magnification corresponding to Figures 24 and 25 on the same 

 plate. In this embryo the caudal flexure attains an unusual degree of curvature — the 

 posterior half of the embryo is hook^shaped. A cervical flexure is lacking, but the cephalic 

 flexure is slightly greater than in any previous stage. There is a yolk stalk, not shown in 

 the drawings that immediately precede this one, and just above the yolk stalk there ap' 

 pears to be a tubular mid'gut. Of interest are the thin roof of the medulla (not seen in 

 any previous stage) represented by the heavily shaded portion of the hindbrain; the 

 closure of the ventral portion of the spiracular cleft; and the large size of the first gill-cleft. 

 The second gill'cleft is of moderate size, and the sites of the future third and fourth gill- 

 clefts are occupied by pharyngeal grooves. If the circular pale spot on the forebrain 

 indicates the position of the eye, then it is nearer the dorsal end of the mandibular arch 

 than it has been in any preceding stage. In this surface view, one cannot be sure of the 

 position of the otic vesicle, but it appears to be dorsal to the first gill-cleft. 



Figure 29, plate II, portrays a surface view of an embryo with about 55 complete 

 somites. The mid-gut is not so far advanced in its development as it is in the preceding 

 figure. The lower two-thirds of the spiracular cleft is closed, and there is a distinct third 

 gill-cleft. The mandibular arch is more prominent than it has been in previous stages. 

 The myelencephalon (medulla oblongata) shows prominent neuromeres. The otic vesicle 

 appears to be dorsal to the first branchial arch. In the last three stages studied, there has 

 been a steady growth of the forebrain. 



Figure 30, plate II, pictures the head and anterior part of the body of a cleared embryo 

 drawn at a higher magnification than that employed for the surface views just considered. 

 The number of somites is unknown, and it is not certain that this specimen is older than 

 the one represented in the preceding figure. Evidently the artist had trouble in getting 

 a clear view of some parts of this large embryo, for the drawing does not show as much 

 detail as one would expect in a figure of this size. There is a slight cervical flexure and the 

 usual pronounced cephalic flexure. The form of the brain has undergone some changes; 

 in particular the telencephalon or secondary forebrain has increased in size. The spiracular 

 cleft is not shown. The otic vesicle is still situated dorsal to the first gill-cleft, though in 

 adult sharks its derivative, the membranous labyrinth, is more closely associated with 

 the spiracular canal; e.g., as in Chlamydoselachus (Smith, 1937, pp. 423 to 430 and 

 Text-figure 82.) 



Figure 31, plate II, portrays a surface view of an embryo with about 74 somites — now 

 represented by myomeres. This embryo is the first to show a fin bud — the pectoral. 

 Rudiments of the first and second dorsal fins, and of a pelvic fin, are recognizable only by 

 comparison with the figures that follow. The tail is bordered by a continuous fin fold 

 out of which will emerge the anal and caudal fins. There seems to be a low fold con- 

 necting the rudiments of all the unpaired fins; but one cannot be sure, from the 

 figure, whether pectoral and pelvic fin rudiments are connected by a fin fold. The 

 yolk stalk is now attached to the pectoral region only, just behind the heart. The spiracu- 



