734 Bashford Dean Memorial Volume 



drawn directly, under low magnihcation, by transmitted light; but in the table in Dean's 

 notebook many embryos are listed as photographed and also drawn. Most of the draw- 

 ings must be interpreted as optical sections, so it is possible that they were drawn from 

 photomicrographs. Such dra^^^-ings show only what can be seen by focussing at a single 

 level, and are sometimes difficult to interpret. It is iinfortunate that, with very few 

 exceptions, the embr^'os from which Dean's drawings were made cannot be found. As 

 an aid to the study of the drawings, I have examined, under both monocular and binocular 

 microscopes, a close series of Elasmobranch embryos, chiefly Squalus acanthias, stained 

 and mounted by me nearly thirty years ago. 



Figure 14, plate I, represents a stage intermediate between Figures 12 and 13. It 

 lacks the high Hghts characteristic of a surface view. Posteriorly, it represents that 

 portion of the thickened margin of the blastoderm adjoining the embryonic shield. Here, 

 by a process of in turning accompanied by a limited amount ot concrescence and a very 

 considerable amount of overgrowth, the thin rim of the early blastoderm has formed 

 a deeper layer, the entoderm, not shov.Ti in the dra'wing. In the angle between the super- 

 facial layer (ectoderm; and the entoderm a middle layer, the mesoderm, is being proliferated. 

 The thickened margin of the blastoderm contains all three germ layers, hence it is some- 

 times called the germinal ring. At the posterior end of the figure a pair of dark zones, one 

 on each side of the mid-line, probably contain axial mesoderm. Anteriorly, a dark zone 

 having the form of an arch represents an optical section through upraised ectoderm at the 

 edge of the germinal shield. The neural groove is out of focus and is not shown. 



Figure 15, plate I, represents a stage sKghtly later than Figure 13. It shows a well- 

 defined notochord, with, its characteristic irregular transverse striations, extending along 

 the mid-Hne of the lower two-thirds of the figure. On each side of the notochord we 

 see the axial mesoderm, very thin and not ready to be cut into somites. Lateral to this, on 

 each side, a broad dark rone represents the inner Hmb of the neural folds. The outer Hmb 

 of the neural folds forms the margin of aU the anterior two-thirds of the figure. The 

 beginning of the fore-gut may be present in this stage, but if so, it is not clearly shown. 

 The figure is possibly a ventral view. 



Figure 16, plate I, is the earliest drawing showing mesoblastic somites. Of these, 

 four pairs are complete. In the head region the neural folds appear asymmetric. On the 

 left side, both outer and inner limbs of the neural folds are well shown, but on the right 

 side the inner limb is incomplete. Evidently the drawing represents an optical section, and 

 the apparent asymmetry of the neural folds is due to a sHght rotation of the embryo on its 

 long axis. The continuation of this rotation will soon bring the embryo to He on its left 

 side. Parenthetically, it may be remarked that the embryo of Torpedo, as figured by 

 Ziegler (1892 and 1902j, tends to He on its right side; but in my whole mounts of Squalus, 

 the embryo Hes on its left side as in Heterodojuus japomcus. In the stage under con- 

 sideration the beginning of the fore-gut, extending forv.^rd beneath the brain as a pocket- 

 Hke portion of the entoderm, is presumably present; but it occurs at a lower level than the 

 structures sho^xTi in the drawing. 



