110 A. p. THOMAS. 



In the last stage, Avhen the embryo is ready to emerge from 

 the shell, it lies slightly curved upon itself at ona side of the 

 egg (v. fig. 4), the remainder of the space being occupied by 

 the fluid remains and refuse of the yolk -spheres. At the 

 anterior end, just beneath the operculum, is a quantity of 

 viscid mucus, which forms a sort of lining or cushion against 

 which the head-end of the embryo is pressed. Around the 

 body of the embryo may be distinguished a bright border, 

 which is formed by the cilia covering its surface ; these cilia, 

 however, can only in exceptional cases be seen in motion before 

 the animal quits the egg. 



2. Free Embryo. — The embryo is now ready to come forth; 

 its movements become more marked, and at length a vigorous 

 extension of the body causes the operculum to fly open, as if 

 moved by a spring. The cushion of mucus pours out, the 

 embryo thrusts the fore part of its body out of the shell, the 

 cilia begin to move instantly the water touches them, and the 

 animal, after a short struggle, succeeds in drawing the whole 

 of its body through the narrow opening of the shell, and glides 

 away with ease and rapidity through the water. Although 

 light has no influence in accelerating development, the embryo 

 itself is very sensitive to it. Thus they congregate especially 

 on the light side of a vessel containing them ; and I have 

 repeatedly observed that, although on removing a vessel of 

 eo^ffs from the darkened incubator in which they were being 

 hatched, not more than two or three embryos could be seen, 

 yet after it had stood in a window for twenty minutes the 

 water was quite nebulous from their presence. 



The form of the free-swimming einbryo is an elongated cone, 

 with rounded apex (fig. 5, Plate II), its average length 013 

 mm., its breadth at the anterior end 0*027. The broader end 

 or base of the cone is directed forwards, and in its centre is a 

 short retractile head-papilla. The whole of the surface, with 

 the exception of the head-papilla, is covered with long cilia, 

 which are borne by an outer layer of flattened ectoderm cells. 

 These cells are arranged around the body in transverse rings, 

 usually five in number, though occasionally six may be 



