of the staf^o is arbitrarily fixed at the period of development when the 

 germ ring encloses four-fifths of the yolk. By the end of the stage 

 the embryo extends about two-fifths of the vray around the yolk, v:ith the 

 cephalic region both thicker and wider than the remainder of the embryo. 



Stage V 



The period of development covered by this stage occupies the final 

 enveloping of the yolk by the germ ring and the closure of the blastopore. 

 The germ ring continues its centripetal gi-oirth until the yolk is completely 

 encompassed by a cellular sheath; the final closure occurs just back of 

 the posterior pole of the embryo, 'wliile the germ ring is completing 

 this closure, there is a thickening all along the ombrj'-o, although the 

 resulting dorsal ridge is still most pronounced in the cephalic region 

 and tapers off posteriorward, so that even by the end of this stage 

 the tail is hardly differentiated from the surrounding tissues, and the 

 embryo itself is a simple, undifferentiated' ridge in which somites are 

 not yet defined. Eggs are considered as belonging to this stage until 

 the optic vesicles arc distinguishable in the head of the developing embryo. 



Stage VI 



This stage begins after the closure of the blastopore when the out- 

 line of the optic vesicles can be seen in the cephalic region of the 

 embryo; it ends when the tail starts to separate from the yolk. The 

 optic vesiclos occupy fully half of the head during this stage; the somites 

 take shape, being evident all along the embryo except in the imr.iediate 

 vicinity of the head and tail; the embryo extends about two thirds of 

 the v;ay around the yolk; the single oil globule in the yolk is a little 

 beyond the posterior end of the embryo and retains this position in the 

 yolk mass subsequently. This stage is of longer duration than any of 

 the preceding three. By the end of the stage, the pupils can be, discerned 

 in the eyes, the embryo is about of equal thickness along its entire - 

 length, v/hile the tail is somcvirhat swollen at its tip. 



Stage VII 



This stage commences v/hon the tail begins to separate from the yolk 

 mass; it covers the early growth of the tail until the free portion is 

 about of equal length to the head of the embryo. Although the demarca- 

 tion of the beginning of this stage is rather definite, its termination 

 is somewhat arbitrarjr. All the remaining stages arc separated primarily 

 on the degree of development of the tail, as it offers the best landmarks 

 on which to base separation into stages. During Stage VII the tail re- 

 mains of uniform thickness along its length; the posterior end is rounded 

 and even appears somei/hat swollen in some individuals; the fin fold is 

 barely discernible, extending aroxond the free portion. of the tail and 

 along the dorsal side of the body to near the region of the head; there 

 is either no differentiation of somites in tht; free portion of the tail 

 or at best a fevj- somites are formed at the base of the tail by the end 

 of the stage; along the rest of the body the, somites stand out sharply 

 when viewed from above. By the end of this stage in most specimens the 

 free tail extends up to, but not beyond, the position of the oil globule, 



149 



