I02 CHIM^ROID FISHES AND THEIR DEVELOPMENT. 



LATE EMBRYOS. 



EMBRYOS FROM THE APPEARANCE OF GILL-OPENINGS TO THE TIME OF HATCHING. 



Four specimens illustrating this period are described on the following pages. 

 The first of these, shown /;/ toto (plate vii, fig. 45), illustrates a stage in which all 

 five gill-slits are clearly shown, but of these only the first has completely broken 

 through, that lying immediately below the auditory vesicle. In front of this the 

 spiracular cleft is faintly outlined. The entire head region is modeled clearly, and 

 the anterior end of the embryo has separated from the blastoderm as far back as 

 the region immediately behind the heart. The tail has greatly elongated and has 

 entirely lost the bulbous terminal which we noted in the earlier embryo. 



The region immediately' adjacent to the embr3'o is divided up into a spongy 

 mass by many blood-vessels; we observe also that the blastoderm has almost com- 

 pletely inclosed the attached yolk mass, a small yolk plug only being visible at the 

 hinder end of the yolk-sac. This condition is shown in plate viii, fig. 48. Here, 

 through the rim of the blastoderm one can faintly see the extent of the submarginal 

 sinus which was noted in the preceding stage. From it now extend many vessels, 

 as indicated in the figure. The region of the yolk plug is figured in plate viii, 

 fig. 48^, as viewed under a dissecting microscope. It shows an interesting condition 

 in connection with the holoblastic behavior of the yolk; for a number of irregular 

 masses are visible, outlined, it appears, by vacuoles, and suggest 3'olk-filled blasto- 

 meres. It will be observed, however, that the contours of the yolk masses are less 

 definite as they approach the irregular rim of the blastoderm. {Cf. fig. 77.) 



Sections of this stage are shown in the adjacent figures. In the first (fig. 78) 

 the mouth (/. c, its hinder portion) and auditory vesicles are traversed; the mouth 

 has not yet broken through nor has the neighboring gill-slit, the hyomandibular. 

 We note that the auditory vesicle is now a thick-walled sac opening broadly at the 

 surface; that a subnotochordal rod is present; that the brain wall in this region (hind- 

 brain) is remarkably thick and asymmetrical, and that the fifth ventricle is corre- 

 spondingly reduced in diameter. A section through the mid-trunk (fig. 79) indicates 

 that in this region the trunk is spread out more widely than in the corresponding 

 or, in fact, in any stage in the shark. The splanchnocoele {spc) is of great size, and 

 its walls, both splanchnic and somatic, contain large spaces. The myocoele is 

 virtualh' obliterated, although its margining cells have not fused across its earlier 

 opening into the gononephrocosle. The last region is not clearly demarked; 2X pn a 

 pronephric tubule appears in the position usual in elasmobranch. At df the early 

 condition of the dorsal fin corresponds closely with that of a shark embryo. 



A second embryo (plate vii, fig. 46) slightly older than the preceding, was one 

 of the specimens received from Dr. Wilbur. It had with it only a small fragment 

 of the blastoderm, and at the time of preservation the embrj'o appears to have 

 turned in a position nearly transverse to its usual one. At this stage the tail 

 protruded widely over the rim of the blastoderm, and it follows, therefore, that, 

 probably as an individual variation, the blastoderm has not as completely inclosed 



