532 
ME. W. K. PAEKEE ON THE STEUCTUEE AND DEVELOPMENT 
A sectional view (fig. 4) throws much further light upon this stage ; and altogether 
we see the condition of the neural axis, the skeletal axis (notochord), the paired sense- 
capsules, the developing visceral arches and branchiae, and the opening visceral clefts. 
The section (fig. 4) not being exactly in the mid line, only exposes the pharyngeal or 
branchial region of the alimentary canal, which, however, was developed at a much 
earlier period ; the oral passage is still incomplete. 
My work here yokes on to that of the embryologist ; the stage which immediately 
precedes this, as well as earlier stages, can be studied in Gotte’s magnificent work on 
Bombinator igneus , for the development of the Amphibian embryo, but still more 
instructively in one of Mr. Balfour’s papers. 
In the paper here referred to (‘ Studies from the Physiological Laboratory,’ pp. 1-20, 
pi. 1 : Cambridge, 1876) the embryo of the Elasmobranch is compared with that of the 
Amphibian. 
The third stage of the Amphibian embryo (fig. 3 C, p. 455) is that which comes in 
directly before my first morphological stage ; this and the other two (A and B) are given 
by Mr. Balfour as “ diagrammatic longitudinal sections of Bombinator igneus, repro- 
duced with modifications from Gotte.” 
This variously shaded woodcut shows what part the three main layers of the blasto- 
derm take in the formation of the embryo. In the earliest stage (3 A) there was a seg- 
mentation cavity, but no alimentary cavity. In the second (3 B) the alimentary cavity 
is formed, but is closed at both ends, and the segmentation cavity is disappearing. In 
the third (3 C) the latter has disappeared, and the alimentary canal is open behind ; 
the neural canal opens into it close to this hinder opening, which is the “ anus of 
Husconi;” it does not correspond with the proper or permanent anal opening*. 
In the third embryological stage the embryo is longer than the diameter of the yolk, 
which has become elongated and flattened, and is enclosed in a layer of epiblast cells, 
the future epidermis. Both the head end and the tail end of the embryo, which is like an 
inverted boat, project beyond the yolk-mass, and, indeed, this mass is now enclosed by a 
layer of mesoblastic cells inside those of the epiblast. The cephalic and caudal ends of the 
embryo are curved downwards ; and there is a layer of mesoblastic cells both above and 
below the neural axis, which is swollen where the cerebral vesicles will soon appear. 
There is no mouth, but at the part below the cephalic end of the embryo the meso- 
blastic layer is deficient. At this part the closed alimentary cavity is very large ; it is 
roofed throughout by hypoblast, which becomes developed into epithelium. 
* Of the “ blastopore ” or anus of Eusconi, Mr. Balfour says : — “ This is the primitive opening by which 
the alimentary canal communicates with the exterior, or, in other words, the opening of the alimentary invo- 
lution. It is a distinctly marked structure in AmpTiioxus and the Batrachians, and is also found in a less 
well-marked form in the Selachians ; in Birds no trace of it is any longer seen. In all those Vertebrates in 
which it is present, it closes up, and does not become the anus of the adult. The final anus nevertheless 
corresponds very closely in position with the anus of Euscoxi.” (‘ A Comparison of the Early Stages in the 
Development of Vertebrates,’ pp. 18, 19.) 
