EMBRYOLOGY AND LARVAL DEVELOPMENT OF TELEOSTEAN FISHES. 4I7 



The quaxitity of protoplasm contained in these eggs is relatively less than that 

 contained in the eggs of the former species and early development advances somewhat 

 more rapidly. The blastodisc is well developed one hour after fertilization. An early 

 stage in the development of the blastodisc is illustrated in figure 17. It is now lenticular 

 in form, but before cleavage occurs it becomes of nearly uniform thickness throughout 



LUCANIA P,\RVA. 



Fig. 20. — Egg with blastodenn of 8 cells. 



Flc. 21. — Egg with blastoilemi of 16 cells. 



the central area and thins out abruptly near the periphery. The first act of cleavage 

 usually occurs within one and one-fourth hours after fertilization. A typical 2-celI stage 

 is illustrated in figure 1 8. Successive stages of cleavage are illustrated in figures 1 9 to 21 . 

 Later cleavage advances relatively rapidly and at 13 hburs after fertilization the 

 germ ring is completely differentiated (fig. 22, or). Figure 23 illustrates a late stage in 



«». 



LUCANIA PARVA. 



Fig. 22. — Egg with blastoderm showing germ 

 ring (gr) fully difierentiated; pp, posterior 

 pole of blastoderm. 



Fig. 23. — Egg with blastoderm showing advanced stage 

 in differentiation of embryonic shield; ea, embryonic 

 area; eea, extra-embryonic area. gr. germ ring. 



the formation of the embryonic shield in which the embryonic axis is already differen- 

 tiated. The blastoderm now covers more than half the surface of the yolk. 



At 24 hours after fertilization (fig. 24) the embryo is well differentiated and the 

 blastopore is closed. At this stage the length of the embryo is less than half the circum- 

 ference of the yolk sphere and is relatively more slender than the embryo of Cyprinodon 

 variegatus at a corresponding stage. 



The later development of the embryo advances relatively slowly. Incubation at 

 laboratory temperature occupies seven to eight days. Before the close of the second 



