



Microns 



40 



Figure 306. — Unfertilized egg of C. virginica centrifuged 

 for 10 minutes at 4,000 times gravity after staining with 

 toluidin b!ue. Dark yolk granules are at the lower 

 (centrifugal) pole while the mass of small inclusions con- 

 sisting of lavender particles and bluish mitochondria are 

 at the opposite end of the egg. 



The chromosomes appear as fine threadlike struc- 

 tures near the nuclear membrane. Using methyl 

 green and pyronin B stains, Kobayashi (1959) 

 found that the nucleolus in the eggs of C. gigas and 

 0. laperousi consists of two parts, the karyosome. 



Microns 



30 



Figure 307. — Ovocyte of 0. laperousi stained with methyl 

 green and pyronin B after Xavashin fixation. Dark 

 globule is the karyosome, light shaded area is the 

 plasmosome. From Kobayashi, 1959. 



shown in solid black in fig. 307, and the plasmo- 

 some, lightly shaded, in close contact with each 

 other. The definite KP axis (Karyosome-Plasmo- 

 some) of the unfertilized egg changes after fertil- 

 ization by the turning of the karyosome around 

 the plasmosome. The existence of this axis in the 

 eggs of other species of oysters has not been 

 described. 



STRUCTURE OF THE MATURE 

 SPERMATOZOON 



The spermatozoon of bivalve mullusks appears 

 under the microscope (Franzen, 1956; Lenhossek, 

 1898; Retzius, 1904) to consist of an oval or 

 round head with a pointed front, a middle piece 

 at the lower end of the head, and a long tail 

 (flagellum) with a narrow "end piece" which is 

 longer than the width of the head. The middle 

 piece consists of fom*, sometimes five, oval-shaped 

 bodies clustered around the tail; and a minute 

 "central granule" or centriole located in the cen- 

 ter at the point of attachment of the tail. The 

 sharply outlined oval bodies are mitochondria; 

 they are strongly osmiophilic and can be deeply 

 stained with rosanilin. The head of the sperma- 

 tozoon is formed by a compact nucleus capped 

 with the apical body or acrosome with a pointed 

 tip (perforatorium), which apparently assists the 

 spermatozoa in penetrating the egg membrane at 

 fertilization (Wilson, 1928). The features listed 

 above may be seen in properly fixed and stained 

 preparations of the sperm of C. inrginica and in 

 live spermatozoa examined with phase contrast 

 oil immersion lenses. In live preparations the 

 nucleus appears to be dark while the acrosome 

 and middle piece are light (fig. 308). The center 

 of the spermatozoon head is occupied by an axial 

 body, a relatively large, light-refracting structure 

 which is separated from the acrosome. 



The dimensions of normal, uncytolyzed sperma- 

 tozoa have been measured by means of an eyepiece 

 micrometer of a light microscope. The head varies 

 from 1.9 M to 3.6 m in length (median value 2.7 ^) 

 and between 1.0 ai and 2 m in width. The tail is 

 from 27 m to 39 m long (median value 36 y). The 

 tails are usually slightly curved; specimens with 

 straight tails are rarely found. 



Electron microscopy reveals much greater com- 

 plexity in the structure of the spermatozoon 

 (Galtsoff and Philpott, 1960). Study was made 

 of small sections of ripe spermary preserved in 

 cold 1 percent osmimn tetroxide buffered to pH 



EGG, SPERM, FERTILIZATION, AND CLEAVAGE 



335 



