370 nSH FOR EXPERIMENTATrON 



Stage 2. Cleavage (Fig. 2) 



Cleavages may be seen only occasionally, and only in eggs preserved in formalin. The 

 cleavage cells are very thin, broad and flat, and since they are not raised above the yolk 

 surface to any visible extent, this stage is poorly distinguished from the previous one. 

 Fig. 2 shows the cleavage stage more distinctly than it actually appears. Using a glass 

 needle and a pair of sharpened watchmaker's forceps, the fertilization membrane can be 

 removed from such eggs while in the saline solution, and the contents left in place. Such 

 a membrane cannot be demonstrated around infertile ova. This fertilization membrane 

 persists throughout the gestation period and is ruptured together with the follicle just 

 prior to parturition. 



Stage 3. Compact Blastula (Fig. 3 ) 



This is the earliest stage which can be identified readily by gross study. The cells are 

 small and tightly packed into a small grayish protoplasmic disc, which is slightly raised 

 above the yolk surface. A segmentation cavity has been described beneath the disc 

 (Hopper, 1943). 



Stage 4. Diffuse Blastula (Fig. 4) 



Gastrulation begins at this stage with the blastodisc flattening out into a thin membrane 

 of cells. The periphery of the blastodisc is uniformly thickened, indicating the region of 

 proliferation and probable involution. 



Stage 5. Early Germ Ring Gastrula (Fig. 5) 



Gastrulation continues during stage 5 with a peripheral spreading of the blastodisc in all 

 directions. The embryonic shield is visible as a widening and thickening of a sector of 

 the rim of the blastodisc. 



Stage 6. Late Gastrula - Early Neurula (Fig. 6) 



The embryonic shield takes on an elongate form and becomes raised from the yolk sur- 

 face, indicating the antero-posterior axis of the developing embryo. The notochord is 

 present, and the anterior end of the neural keel can be seen. The nerve cord is formed 

 from a solid core of invaginating tissue; the neurocoele appearing after invagination is 

 completed, as seen in sectioned material. This type of neurulation is typical of teleosts. 



Stage 7. Late Neurula (Fig. 7) 



The germ ring at this stage is somewhat below the equator and the embryo has become 

 further elongated. Since elongation takes place principally in the posterior portion, a 

 region roughly corresponding to the dorsal blastopore lip of amphibian gastrulae, the 

 anterior end of the embryo lies in much the same position as did the original embryonic 

 shield of stage 5. 



The neural keel has invaginated throughout the greater length of the embryo, and a neuro- 

 coele is present in the anterior one-fourth. 



Stage 8. Head Fold (Fig. 8) 



A prominent head fold is present by stage 8. The neurocoele is open for about the an- 

 terior half of the length of the embryo. The optic buds are present and attached to the 

 short, thin stalks, and they are, at this stage, without a cavity. Two pairs of rather dif- 

 fuse somites are evident, but there is considerable variation in the time of their first ap- 

 pearance. Somites sometimes appear as early as stage 7. 



