204 W. G. V<m]\^arne — Euihryologi/ of EufitylocJivs. 



creases in size and becomes circular. It lies in or attached to the 

 ectoderm but does not reach the surface. A day or so later another 

 spot appears in a similar position on the right side. This also in- 

 creases in size, but for many days that on the left side is conspicu- 

 ously larger (Fig. 32). Occasionally an embryo presents an abnor- 

 mality in regard to the number and arrangement of the pigment 

 spots, but so far as I have discovered, the earlier appearance and 

 larger size of that on the left side is invariable. Except for this the 

 embryo is bilaterally symmetrical, if Ave leave out of account the 

 large yolk-masses which change their position as the animal turns 

 and contracts. 



Later the surface of each eye-spot becomes concave, and in the 

 concavity a ti*ansparent vesicle is developed. 



By the time the eyes have appeared, the ectoderm in the depres- 

 sion between the ventral lobes, mentioned above, can be seen to be 

 deeply bent inward into a funnel-shaped cavity extending upward 

 and somewhat forward. At the anterior end of the embryo, almost 

 between the eyes, a few stiff hairs grow out in a group and generally 

 adhere so closely together that the group appears like a single coarse 

 flagellum. Later a similar, but at first shorter, group appears at the 

 posterior end. All these structures are developed by the time the 

 embryo escapes from the egg-membrane. They can, however, be 

 much more easily observed after this has occurred, which may be 

 anywhere from ten days to two Aveeks after the Qgg is laid, or occa- 

 sionally longer. Toward the last the egg-membrane becomes very 

 thin, and with the most careful handling many embryos will be pre- 

 maturely released. For several days before the embryo escapes its 

 movements are very active, and if the membrane is torn it swims 

 rapidly away by means of the cilia. Contrary to what Girard saj^s. 

 I find that the embryo almost always SAvims Avith the anterior end 

 directed forwards, but it keeps turning about its longer axis so that 

 the ventral surface is often upward. 



Fig, 32 shows an embryo which Avas draAvn as it was endeavor- 

 ing to escape through a small hole torn in the egg-membrane, and 

 Fig. 33 one in about the same stage removed from the membrane 

 and seen from one side. These Avere about eight days old. But 

 little further change takes place up to the time of hatching. In the 

 course of the first fcAV days after hatching the embryos assume the 

 curious form shown in Fig. 34, though there does not seem to be 

 any considerable change in the internal structure during the interval. 

 Fig. 35 shows a still later stage ; the external form is similar to that 



