The Embryology of Heterodontus japonicus 741 



than in the preceding drawing. The brain shows differentiation into the primary vesicles 

 (forebrain, midbrain, hindbrain) and there are indications of a secondary division of the 

 hindbrain into myencephalon and metencephalon. The bulge at the side of the forebrain 

 represents an early stage in the formation of the optic vesicle. 



I have found in Dean's collection of microscopic slides a total mount of an 18'somite 

 embryo labelled ''Cestracion.'' This appears to be the specimen from which Figure 20, 

 plate II, was drawn. The embryo is slightly overstained, but corresponds to the drawing 

 in every respect save that the small round black spot in the region of the neurenteric 

 canal is lacking, and the triangular dark area (mesodermal?) at the anterior end of the 

 fore-gut is not so sharply defined. In view of the scarcity of information concerning si2;es 

 of the embryos represented in the plates, it is interesting to note that this iS-somite 

 embryo, measured on the slide, is 3.5 mm. (about one-eighth inch) long. 



Figure 21, plate II, portrays in surface view an embryo of about 24 somites (one side 

 only). This figure, and the one immediately following, appear to be drawn at a magnifi- 

 cation lower than that employed for the cleared specimens that precede them. The 

 body of this embryo leans to the left, while the head is turned slightly to the right. 

 The tail bud projects for some distance beyond the posterior rim of the blastoderm, and 

 the head is entirely free from the underlying structures. In this embryo both cephalic 

 and cervical flexures have almost reached their maximum. The right optic vesicle and lens 

 are faintly indicated. The pronounced bulging in the hyoid region and that dorsal to 

 the midbrain are probably abnormal. 



In Figure 22, plate II, a surface view, only 20 somites are readily visible; but in the 

 caudal region four or five more are faintly indicated, making a total of about 25. In 

 addition, there is an incomplete somite, probably degenerating, at the anterior end of the 

 series. This embryo appears normal save for the presence of a large bulge of the ectoderm 

 over the midbrain and a lesser bulge of the same kind dorsal to the anterior (incomplete) 

 somite. For the first time in this series, we see something like a yolk stalk — in this stage 

 very short and thick. The rudimentary eye is decidedly larger than in the preceding 

 drawing, and there is more differentiation in the branchial region. Of the visceral arches 

 the mandibular, hyoid and first branchial are recogniz;able; of the branchial grooves, the 

 spiracular (Y-shaped) and first branchial. The forebrain bulges a little dorsally. 



Figure 23, plate II, represents a cleared embryo with at least 26 somites. It seems to 

 be drawn at a slightly higher magnification than the two preceding figures which are 

 surface views. It can scarcely be said to possess a yolk stalk since it is attached to the 

 yolk mass along almost the entire length of the body proper. Vitelline arteries and veins 

 are faintly indicated on the extra-embryonic blastoderm near the embryo. The optic 

 vesicle shows a distinct chorioid fissure. Dorsally, in the region of the hindbrain, there 

 is a somewhat indistinct otic vesicle. The blister-like elevation of the ectoderm, dorsal to 

 the midbrain and to the anterior part of the hindbrain, is probably abnormal. Some 

 neuromeres occur in the myelencephalon, immediately behind the otic vesicle. Between 

 the foregut and the diencephalon there is a straight bar of tissue which may represent the 



