242 RICHAED ASSHETON. 



DESCRIPTION OF PLATES 23 & 24, 



Illustrating Mr. Richard Assheton's paper *^0n the Growth in 

 Length of the Frog Embryo." 



Complete List of Retebence Letters. 



A. Anterior end. a a. Small cells in floor of segmentation cavity, bl. d. 

 Dorsal lip of blastopore. bl. v. Ventral lip of blastopore. D. Dorsal 

 surface, exo. Exovate. P. Posterior end. sg. Segmentation cavity, y. 

 Dorsal lip of blastopore, x. Floor of segmentation cavity, z. Yolk-cells, 

 which will be overgrown by dorsal lip of blastopore, y. p. Yolk-plug. 

 X . Spot at which the blastopore commences. 



PLATE 23. 



Pig. 1. — a — d. Ovum pricked in centre of white between the developing 

 lips of the blastopore. 



Pig. 3. — a — d. Course taken by a natural mark on the white pole. 



Pig. 3. — a — g. A mark was made near the centre of the white pole. 



Fig. 4. — a, b. Ovum was marked in centre of blastopore when first out- 

 lined. 



Pig. 5. — a — d. Three embryos marked at the cross in 5«. In ^ the mark 

 was on the side ; in e, ventral to blastopore ; in J, in which it was very 

 indistinct, at the side of the blastopore. 



Fig. 6. — a, b. This embryo was marked at the sides of blastopore. 



PLATE 24. 



Pig. 7. — Prog embryo before appearance of blastopore. The white (or 

 posterior) pole is placed to the right of the observer. 



Pig. 8. — Prog embryo at the time of the commencement of the blasto- 

 pore, which is shown as a dark crescentic groove. The dorsal cap represents 

 that part of the epiblast which will form the anterior part of the neural 

 plate. 



Fig. 9. — Frog embryo at the moment of the completion of the ventral lip of 

 the blastopore. The dorsal lip has grown over the white pole through an arc 

 of 50° to 60°. The dotted line indicates that part of the embryo supposed 

 to be derived from the secondary area of proliferation. The arrows indicate 

 from which portion of the rim the respective parts have been formed. 



Pig. 10. — Frog embryo at the time of first appearance of neural plate, 

 visible only in sections. The blastopore is much reduced. The ventral lips 



