The Embryology of Heterodontus japonicus 



753 



Since the young fish pictured in Figure 84, plate VII, is only two weeks older, it is best 

 considered in this section. Dean's observations are here quoted from his typed manuscript. 



A stage shortly before hatching is shown [Text-figure 59]. The young fish is coiled 

 compactly, the fins wrapping around the body, the head being below [i.e., toward the small 

 end of the capsule] , the trunk bent into a loop and the tail continued so that it approaches 

 the larger end of the capsule. The snout lies close to the breathing apertures of the smaller 

 end of the capsule, and the gill-openings are not [very] distant from the right and left aper- 

 tures of the capsule's larger end. These apertures at such a stage are large, the weathering of 

 their margins having progressed to such a degree that a considerable current of water may 

 be circulated through the capsule from the smaller to the larger end. The circulation is 

 effected by the young fish, for in the partly opened capsule one may see with what strong 

 muscular effort the fish is compressing and expanding its gill-pouches, drawing the water in 

 through its mouth at the smaller end of the capsule and ejecting it in the opposite direction. 



Text-figure 60. 



Outline drawing showing the attitude of a newly hatched Heterodontus japonicus, with its head upraised and 



partly out of water as if seeking to escape from the aquarium in which it was confined. 



From a drawing left by Bashford Dean. 



In a single instance the act of hatching was observed. An egg was brought in which 

 was curiously light in weight, its walls papery and studded with barnacles: at first sight it 

 seemed empty, but an examination showed that the larger end of the capsule had not opened. 

 On April 4, it was placed in a laboratory aquarium: four days later, happening to take it in my 

 hand, I felt it suddenly vibrate, as though it enclosed a young fish which had been alarmed by 

 my touching it. This movement lasted a few seconds, then the fish suddenly appeared. The 

 hatching took place so quickly and unexpectedly that its details were not followed. The valve 

 opened and closed, and there was a young fish swimming about in the aquarium. It had 

 emerged tail foremost, that was about all that was definitely noticed. ^ 



A student of animal behavior would have been interested in this newly hatched fish. 

 For it showed the most finished instincts. It swam around the aquarium actively for about 

 half a minute, breathing quickly and expanding its gills. It had from the beginning the move- 



' In Dean's original notes it is stated, in his own handwriting, that the young fish had emerged head foremost. Considering that in 

 this stage the yolk mass had almost entirely disappeared, it appears probable that the fish would be able to reverse its orientation within 

 the capsule, and thus either end might escape first from the capsule — presumably from its broad end. 



