Normal Plates of the Development of Squalus acanthias. 9 
the mid- and fore-brain vesicles. The mid-brain vesicle is expanded laterally. The hind-brain is a little 
dilated, as is best seen in dorsal view. The entire outline of the trigeminal crest cannot be distinguished, 
but its posterior boundary appears as a raised line passing from the dorsal zone of the hind-brain to the 
upper part of the mandibular arch. The facial-acoustic crest appears as a faintly outlined band Iying 
anterior to the otic pit and merging in the hyoid arch ventrally. The otic pit is a broadly oval depression 
with prominently raised walls. The glossopharyngeal crest forms a raised band extending from the posterior 
wall of the otic pit into the third branchial arch. The vagus crest is separated from the glossopharyngeal 
crest by a broad groove. Its ventral border is indistinct. The first and second gill slits are broken through. 
The first slit is an elongately oval, the second an almost circular opening. There is a raised oval plate 
over the third gill pouch. The mandibular arches are seen to be in contact in the oral region. The posterior 
thirds of these arches are bent so that they lie in the antero-posterior plane of the body. The outline of the 
heart is barely visible through the walls of the pericardial cavity. Thirty-five or -six pairs of somites are 
visible externally. The Wolffian duct can be seen as a narrow ridge below the eleventh to sixteenth somites 
inclusive. A distinct median ridge extends from the middle of the trunk region along the back, around the 
j tail, and for 0.8 mm. along the ventral median line. 
j Although this embryo measures 0.6 mm. less in length than the preceding one of 5.8 mm., it is 
; considerably farther developed. The chief evidences of this advance are: the more complete separation of 
j { the embryo from the blastoderm; the greater flexure of the head, and prominence of the brain vesicles; 
the breaking through of the two anterior gill slits; the deepening of the otic pit, and the farther devel- 
opment of the median fold. The difference in length is due mainly to the marked tail flexure of the 
older embryo. 
Fig. 21. Embryo 6.2 mm. in length. Normal plate series No. 21. H.E.C. 1637. Lateral view. X 8. 
The embryo is turned with the left side toward the blastoderm, to which it is attached by its middle 
third. The back line is almost straight. The mid-brain, which extends far forward, is bent at right angles 
to the line of the back and the anterior part of the head is bent downward at an angle of 30° to the mid-brain 
region. The tail is markedly flexed. The optic vesicle is almost circular, and projects prominently from 
the side of the head. At the center of the prominence the lentic plate is seen as a faintly outlined circular 
elevation. Above the optic vesicle is seen a faint arched line — the base of the mid-brain. Behind this, 
and still above the optic vesicle, is a small rounded swelling caused by the cephalic end of the mandibular 
somite. The roof of the hind-brain is semi-transparent. The opening of the otocyst is lenticular in outline. 
Two gill slits open to the pharynx. The third gill cleft is prominent, and the fourth is faintly indicated. 
The heart tube can be seen through the thin pericardial wall. Forty-four somites can be counted externally. 
There is a slight tail knob. 
1 | The more evident changes shown in this embryo, as compared to the preceding specimen figured, 
. 5.2 mm. in length, are: the absence of the dorsal flexure; the posterior flexure of the fore-brain region; the 
| more complete closure of the otocyst; the appearance of the lentic plate; the increase in number of gill 
clefts; and the increase in length, and further flexure of the tail. 
Figs. 22, 22a, 22b. Embryo 7.5 mm. in length. Normal plate series No. 22. H.E.C. 1503. Lateral, 
dorsal, and ventral views. X 8. 
The yolk stalk has an antero-posterior diameter of 1.5 mm, There is a very slight dorsal flexure 
above it. The head is bent squarely at right angles to the pharyngeal region. The fore-brain is a little 
Normentafeln zur Entwicklungsgeschichte der Wirbeltiere. XII, 2 
