Normal Plates of the Development of Squalus acanthias. 7 
Fig. 16. Embryo of 40 mm. Normal plate series No. 16. H.E.C. 930. Lateral view. X 8. 
The embryo lies with the left side toward the blastoderm. The head is bent at a little more than 
right angles to the body. The anterior fourth of the embryo is free from the blastoderm. The optic 
vesicle forms a prominent projection from the side of the fore-brain. The medullary canal is closed 
completely, except for a small neuropore. The premandibular somite forms a prominent oval swelling 
above the optic vesicle. The first and second gill pouches are indicated externally by oval swellings in 
the pharyngeal region. The first pouch is a little more prominent than the second. Seventeen somites 
are visible externally. The tail is free from the blastoderm for 0.5 mm. or one eighth of the length of 
the embryo. 
The most striking change between this and the preceding embryo is the complete closure of the 
incisura neurenterica and the separation of the tail from the posterior rim of the blastoderm. The head is 
bent at a much greater angle with the body, and the medullary folds are now fused over the hind-brain. 
The premandibular somite is much enlarged. A second gill pouch has appeared and the first gill pouch 
u 
is more prominent. 
. Fig. 17. Embryo of 3.8 mm. Normal plate series No. 17. H.E.C. 1498. Lateral view. X 8. 
The embryo lies with the right side toward the observer. The anterior two-fifths and the posterior 
one-fifth are separated from the blastoderm. The dorsal back line is a little concave. The pharyngeal 
region and tail are bent somewhat ventrally from the back. The head is bent ventrally from the pharyngeal 
region at an angle of 45°. The optic vesicle forms a prominent oval elevation, the center of which is 
slightly depressed. The otic plate is a little depressed. The mandibular somite appears as a prominent 
wedge-shaped swelling above and a little behind the optic vesicle. It can be followed down the entire 
length of the mandibular arch. The trigeminal nerve crest forms a broad plate lying dorsal to the man- 
dibular somite and the first gill cleft. The facial nerve crest forms a distinct ridge extending from the 
hind-brain to the middle of the hyoid arch. Behind the facial crest the glossopharyngeal-vagus crest 
appears as a small raised triangle over the side of the hind-brain. { 
The first and second gill slits are seen as oval thickenings, each of which is depressed centrally. 
The first gill cleft is nearly twice the length of the second. The heart appears as a rather indistinct 
straight tube Iying in front of the anterior wall of the yolk stalk. Twenty-eight somites can be counted 
externally. The Wolffian duct forms a small ridge lying below the seventh, eighth, and ninth somites. 
As compared with the preceding embryo the noticeable features are: the further separation of the 
embryo from the blastodisk; the prominence of the nerve crests and gill clefts in the head region; the 
increase in number of somites visible externally; and the appearance of the Wolffian duct. 
Fig. 18. Embryo 4.8 mm. in length. Normal plate series No. 18. H.E.C. 1398. Lateral view. X 8. 
The embryo lies with the left side towards the blastoderm. It is connected with the blastoderm for 
a little more than its middle third. There is a distinct dorsal flexure above the attached portion of the 
H embryo. The head is bent at an angle of 60° to the pharyngeal region. The tail is bent slightly ventrally. 
A very faint depression marks the line of division between the fore-brain and mid-brain. The optic vesicle 
forms an oval protuberance from the fore-brain; this protuberance is depressed centrally. The outline of 
the trigeminal nerve crest, with its thalamic and mandibular branches, can be seen above the optic vesicle. 
The facial nerve crest forms a prominent ridge extending from the dorsal zone of the hind-brain nearly to 
the base of the hyoid arch. The otic pit appears as a shallow depression posterior to the upper part of 
