constituent cells extend symmetrically along both 

 sides of the body. Each group gi-ows anteriorly, 

 surrounded by vesicular connective tissue of the 

 visceral mass, and forms a system of profusely 

 branching tubular follicles. The fusion of the 

 branches along the dorsal side obliterates any rem- 

 nants of the paired origin of the gonad. 



The microscopical structure of the gonad varies, 

 depending on the age of the oyster, degree of ma- 

 tiu'ity, season, and environmental condition. In 

 Ostreidae and in some other pelecypods (Pecten, 

 MytUus, Volsella) the follicles of a fully developed 

 gonad consist almost entirely of primitive sex cells 

 (gonia) at various stages of development with only 

 a few minute follicular cells between them. Be- 

 cause of the absence of a capsule or membrane 

 around the gonad, the sex cells are in direct con- 

 tact with the surrounding tissues (fig. 269). It 

 may be assumed that the role of follicular cells in 

 the growth of the gonad of oysters is insignificant 

 and that the gametogenic cells obtain their nour- 

 ishment directly from the connective tissue which 

 surrounds them. In Mya, Teredo, Bankia, and 

 other Adesmacea the follicular cells are large and 

 function as accessory nutritive cells. 



In immature oysters and in the adults, that had 

 completed the spawning period, the germinal 

 epithelium consists of undifferentiated sex cells, 

 some at the early stages of gametogenesis. Their 

 sex can be recognized only by careful cytological 

 examination. As the folhcles grow and ramify 

 they spread throughout the surrounding layer of 

 connective tissue. The maturing sex cells inside 



Microns 



60 



Figure 269. — Terminal portion of the follicle of a gonad 

 of C. virginica at early stage of development (male 

 phase). The follicle is surrounded by a large mass of 

 connective tissue. Redrawn from C'oe, 1936a. 



Figure 270. — Cross section of one surface follicle in a fully 

 developed ovary of C. virginica. Bouin, hematoxylin- 

 eosin. 



them midtiply, grow, and fill up the lumen (fig. 

 270). The foUicles near the surface of the gonad 

 are distinctly different. Their outer walls facing 

 the body surface are fined with ciliated epithelium, 

 and only the inner sides of the follicles retain the 

 germinal cells (fig. 271). This differentiation of 

 the germinal epithelium into two distinct types is 

 probably common to all species of oysters. The 

 follicles lined with ciliated cells, function as the 

 genital canals through which mature sex cells are 

 moved by ciliary action. They were first de- 

 scribed by Hoek (1883) for 0. edulis and subse- 

 quently were found in 0. lurida and in several 

 species of Crassostrea. 



Since the transformation of germinal cells into 

 ova and sperm is a gradual process which does 

 not involve all the cells of the germinal epithelium 

 at the same time, numerous undifferentiated or 

 so-called residual cells are usually found along 

 the inner periphery of a follicle. Some of them 

 can be found even in a fully developed gonad 

 (fig. 270). 



The bulk of a functional ovary is made up of 

 fully developed ova which fill up the lumina of 



300 



FISH AND WILDLIFE SERVICE 



