154 



Fig. 3. Young cellular tissue of the shell of Ostrea edulis, with cytoblasts. 



Fig. 4. Adult cellular tissue of Ostrea edulis. 



Fig. 5. Haversian canals in the membranous tissue of the shell of Ostrea edulis. 



EXPLANATION OF PLATE XVII. 



Fig. 1. A portion of the periostracum of Lymnea stagnalis, with oval cytoblasts. 

 Fig. 2. Periostracum of the same shell, exhibiting the linear arrangement of oblong 



cells, and the mamillated outer membrane at a. 

 Fig. 3. Some of the oblong cells represented in fig. 2, magnified 550 linear, to show 



the minute globular bodies that they contain. 

 Fig. 4. A portion of the mamillated outer membrane of the same shell, seen with a 



power of 1020 linear. 

 Fig. 5. and 6. Two of the oval nucleated cytoblasts represented in fig. 1, seen with a 



power of 550 linear. 

 Fig. 7. A portion of the periostracum of Solen vagina, exhibiting one of the broad 



bands of cells and cytoblasts upon its surface. 

 a. The front of the band. b. The back. 

 Fig. 8. Part of one of the wounded spots in the periostracum of Solen vagina. 



a. New basement membrane, with primitive vascular tissue. 



b. New layers of cellular tissue, advancing from the circumference to- 

 wards the centre of the wound. 



EXPLANATION OF PLATE XVIII. 



Fig. 1. Represents a small portion of the edge of one of the wounded spots in the 

 periostracum of Solen vagina, with a power of 1020 linear. 



a, a. Bifurcating canals excavated in the healthy periostracum surround- 

 ing the wounded part. 



b, b. New cellular structure within the margin of the wound. 



Fig. 2. Primitive vascular tissue of the new basement membrane of the wounded pe- 

 riostracum of Solen vagina, in nearly a complete state of development, ter- 

 minating at one end with large nucleated cytoblasts. 



Fig. 3. A small plexus of primitive vascular tissue, in an early stage of formation. 



a, a, a, a, a, a. Large nucleated cytoblasts intermixed with the smaller 

 ones of which the tissue is principally composed. 



Fig. 4. Primitive vascular tissue of the new basement membrane completely developed. 



a. A nucleated cytoblast in active condition, terminating a branch in 

 course of development from its under surface. 



b. A young branch terminated by a nearly exhausted cytoblast. 



c. Part of the vessel filled with the yellow gelatinous substance. 



Fig. 5. Linear arrangement of large nucleated cytoblasts in the earliest stage of the 



development of vascular tissue. 

 Fig. 6. Primitive vascular tissue, with branches in different stages of development. 

 Fig. 7. Yellow fasciated tissue of the inner surface of Venus decussata. 

 Fig. 8. White fibrous tissue of Venus decussata. 



