926 Dr Allen Thomson on the Vascular System 



i. 5. & C. The heart of the same animal at more advanced periods. 

 7. The foetus of the Sahnon, with the yolk-bag and veins ramified on 

 it (from Ellis). 



PLATE III. 

 .3. Reptiles. 

 Frog. 

 Fig. ,1. The ovum of the Frog, natural size. 



2. Section of the ovum, shewing the manner in which the germinal 



membrane surrounds the yolk, magnified 6 diameters. 



3. (From Prevost and Dumas) Section of the ovum, and the furrow 



formed by the primitive folds of the spinal canal. 



4. (From do.) A and B, The primitive trace and primitive folds. 



5. (From do.) A, The spinal canal beginning to close. B, Side view 



more advanced. 

 G. (From do.) Side view of the foetus more advanced. 



7. (From do.) Posterior view, the spinal canal closed. 



8. & 9. Sections of the tadpole, shewing the inode in which the layers 



of the germinal membrane are disposed. 



10. Tadpole in the egg at the fifth day after being laid, natural size. 



11. & 12. Anterior and posterior views of the tubular heart of this tad- 



pole, magnified. 

 13. & 14. Hearts of tadpoles in which the external gills were nearly 

 perfect. 



15. Tadpole when the external gills begin to lessen. 



16. The heart of this tadpole. 



17. Heart of the tadpole a short time before the transformation takes 



place. 



Salamander. 



Fig. 1. Ovum of the Aquatic Salamander, natural size. 



2. This ovum magnified. 



3. Section of the ovum and germinal membrane. 



The remaining figures indicate the development of the tadpole, and for- 

 mation of the heart, corresponding with those in the Frog. 

 Fig. IC. (From Funk) Shews the yolk and vascular network of the land sala- 

 mander. 

 17. & 18. (From the same) Shew the yolk-bag at an advanced period 



in this animal. 



Lizard. 



19. (From Rathke), The foetus of the Lacerta agilis. 

 4. Mammalia. 



Fig. I. (From Baer) Ovum of the dog (probably eight days after impreg- 

 nation), with the cicatricula on its surface. Fig. 1. Natural size. 

 II. (From Prevost and Dumas) Ovum of the dog, eighth day. Fig. 2. 

 Natural size. 

 III. Ovum which I found in the rabbit on the sixth day. The cicatri- 

 cula is seen on the surface of the yolk, magnified 6 diameters. 

 Fiif. 3. Natural size. 



