EXPLANATION OF PLATES X & XI, 



Illustrating Mr. Sedgwick's Paper on the " Origin of Meta- 

 meric Segmentation/' 



Complete List of Reference Letters, 



A. Auus. a. Anterior end of young embryo. A. P. Abdominal pore. 

 B. Body-wall. C. H. Cerebral hemisphere. C. Longitudinal canal connect- 

 ing pouches of archenterou. B. Ectoderm. G. Gill pouch. H. Heart. 

 K. Nephridium, K. B. Longitudinal duct of Nephridia (segmental duct). 

 JI. Mouth. M. A. Coalesced part of primitive mouth. ME. Hesenterou. 

 me. Edge of mesenteries. M. S. Mesoblastic Somites. N. Nervous sys- 

 tem between mouth and anus. iV'. Prseoral part of nervous system. N^. 

 Postanal part of nervous system. N. C. Neural canal. Ke. Posterior open- 

 ing of neural canal. 0. External openings of Nephridia. P. Pouches of 

 archenteron. P. Prseoral lobe. Py. Anterior opening of neural canal. 

 Si. Siphonoglyphe. Si'. Upper end of siphonoglyphe projecting beyond 

 general edges of lips. St. Wall of stomodaeum. 



Eig3. 1 — 5. — Five young embryos of Peripatus capensis, ventral view. 

 From drawings by Miss Balfour, a. Denotes the anterior extremity. 



Fig. 1. — Youngest embryo, mth slightly elongated blastopore. 



Fig. 2. — Embryo, with three somites and elongated blastopore. 



Fig. 3. — Embryo, with five somites. The blastopore is closing in its 

 middle portion. 



Fig. 4. — The blastopore has completely closed in its middle portion and 

 given rise to two openings, the future mouth and anus. The primitive streak 

 is deeply grooved. 



Fig. 5. — Embryo, with about thirteen somites; flexure of hind part of 

 body commenced. The remains of the original blastopore are present, as the 

 mouth placed between the second pair of somites, and the anus placed on the 

 concavity of the commencing flexure of the hind part of the body. 



Fig. 6. — Stomodaeum of Peachia, laid open so as to show the siphono- 

 glyphe. This figure was very kindly drawn for me by Mr. W. ¥. R. Weldon. 

 T. Tentacles. St. Wall of stomodaeum. Si. Siphonoglyphe. Si'. Upper end 

 of siphonoglyphe, projecting beyond the general edges of the lips. B. Body- 

 wall, me. Edge of mesenteries. 



