EGGS AND YOLK-SAC LARVAE OF YELLOWFIN MENHADEN 



97 



EARLY- STAGE EGG 



The perivitelline space developed and widened 

 witliin 15 minutes after ova and sperm were mixed 

 in sea water. If the ova and sperm were mixed 

 in the absence of water, the perivitelline space 

 was not readily apparent until after sea water 

 iiad been added. Unfertilized and fertilized eggs 

 from the same lot, one hour after the sex products 

 were mixed, are shown in figures 4 and 5. 



Early cleavage was rapid, and a layer of cells 

 was formed by the 7-hour stage (fig. 6). Con- 

 tinued cell division resulted in the formation of a 

 dome-shaped blastodermal cap on the yolk 

 (fig. 7) after 12 hours. Eggs collected from the 

 plankton (fig. 8) showed the blastodermal cap 

 covering nearly one-third of the yolk. These 

 late blastula were estimated as 14 hours old. 



r"^. 



Figure 4. — -Unfertilized egg of yellowfin menhaden. 



Figure 6. — Seven-hour stage witli earlj' cleavage. 



Figure 5. — One-hour stage with perivitolhne space. 



Figure 7. — Twelve-hour stage with blastodermal cap. 



Some of the early stages showed yolk diffusion 

 into the perivitelline space (fig. 9). This was 

 assumed to be due to meciuinical rupture of the 

 yolk membrane during the handling and preserva- 

 tion of the eggs, since yolk encircled by the blas- 

 toderm in later stages did not appear to be 

 ruptured (figs. 10, 11, 12, and 15). 



At the late blastula stage tiie blastodermal cap, 

 now known as the embryonic shield (fig. 9), had 

 developed. The early embryo could be seen as a 

 medial thickening of the shield. The peripheral 

 cells continued to spread over the yolk surface. 



The early neurula marked tiie end of the early- 

 stage egg (fig. 10). The developing embryo, with 

 a discernible head and several myomeres, became 



