582 EXPERIMENTAL FISH EMBRYOLOGY 



DEVELOPMENT OF THE PLATYFISH, PLATYPOEC I LUS MACULATUS 



DESCRIPTION OF NORMAL DEVELOPMENT STAGES 



Stage I. Mature Ovum (Fig. I) 



The mature Infertile ova, after the yolk has teen deposited, average 1.5 nun. In di- 

 ameter. They are of a clear yellow color with peripherally arranged fat globulea of var- 

 ious sizes. These globules vary somewhat in size and numher depending on the individual 

 ovary. However, the eggs in any one ovary are all very similar in this character. When 

 the egg is damaged, the globules are found to be adherent to the peripheral membrane; they 

 are composed of a colorless fluid somewhat less viscous than the yellow colored matrix. 



The germinal vesicle cannot be seen in the living egg, but it has been demonstrated 

 by Hopper (19^+5) to be peripheral in position in sectioned material. 



The vitelline membrane is probably present since a fertilization membrane is subse- 

 quently demonstrated. No tertiary membrane, such as is found in oviparous species, is 

 present around the egg. 



Immature eggs appear to be more opaque than mature fertilized ones. It may be that 

 this change takes place at fertilization as it does in Fundulus ( Oppenheimer, 1957), but 

 in this viviparous species it is difficult to substantiate. 



Staqe 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 re- 

 moved 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 per- 

 sists 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, 191^5 )• 



Stage 4. Diffuse Blastula (Fig. n) 



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

 brane 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 



