26 ' ROYAL SOCIETY OF CANADA 



EXPLANATIONS OF THE FIGURES ON PLATE. 



The sections were projected and carefully outlined on the drawing paper 

 by means of the Zeiss Epidiascope and 20 mm. micro-planar, at such dis- 

 tances as to give an enlargement of 102 for figures 1 to 9, and 116 for figure» 

 10 to 23. 



Subsequently, the drawings, which were made by Mr. J. R. G. Murray, 

 student in biologj-, University of Toronto, were reduced rather more than 

 one-third, so that the magnification is respectively 63 and 72. 



Figs. 1-i, — Xos. -i, 5, 6, and 8, of the series, througli the fore- 

 brain. 



Fig. 5, — No. 12, through the anterior blind ends — ph. — of the 

 pharynx. Ov. and ov' the right and left optic vesicles of the right 

 component. 



Fig. 6, — Xo. 19, through the stomatodaea of both components 

 and the diencephalic region; round the composite pharynx are grouped 

 eight arteries; two ventral, and two dorsal aortae on each side. 



Fig. 7. — No. 33, through the mesencephalon. Ventrad of the 

 pharynx are the two aortic bulbs ; dorsad, the median dorsal aortae have 

 united into a single vessel; re, ectodermic recess under the head. 



Figs. 8, 9, and 10.— Nos. 47, 55, and 67, respectively, through the 

 fifth, seventh and eighth, and ninth nerves. 



Fig. 11, — No. SO, through the second intorsomite. The median 

 dorsal aortœ have given place to a mass of mesoderm. 



Fig. 12, — No. 126, behind the last somite. The chords are gain- 

 ing in size, and the mesodermic mass diminishing. The rudiment of 

 the Wolffian body is seen in this and in Fig. 13. 



Fig. 13, — -No. 131, the chordae have fused. 



Figs. 14 and 15, — Nos. 150 and 154, the chorda and the wall of 

 the neural groove gain in size. 



Fig. 16, — No. 160, the beginning of the fusion betAveen the floor 

 of the neural groove and chorda. 



Figs. 17, 18, 19 and 30,— Nos. 164, 168, 171 and 175, respectively, 

 show the progressive fusion of the neural wall, chorda, mesoderm and 

 entoderm. 



Figs. 21 and 32, — Nos. 181 and 1S6, are through the hinder end 

 of the primitive streak. The former shows traces of an oblique fissure. 



Fig. 33, — No. 196, shows the nature of the mesoderm behind the 

 primitive streak. 



