30 BERTRAM G. SMITH. 



Observations in the field were necessarily discontinued at this 

 stage, but living material was transported to the Zoological Lab- 

 oratory of the University of Michigan. The temperature during 

 transportation was regulated with ice. After their arrival at the 

 laboratory the embryos were kept in filtered water aerated with 

 an aspirator, in dishes partly immersed in cool running water. 

 The loss both during and after transportation was very slight. 



Four Weeks Embryos. — Embryos a month old have attained 

 a length of about 20 mm. The eyes are prominent and pig- 

 mented ; the entire dorsal side of the body is well covered with 

 black pigment, though the pigment is far from being as dense as 

 in an Amblystoma or a frog at a corresponding stage of develop- 

 ment. Lateral line sense organs can be distinguished. The three 

 pairs of external gills are well fringed and pink with blood ; the 

 blood corpuscles are large and may be easily observed with a 

 hand lens. The vitelline vein, now very prominent, has shifted 

 to a position along the lower part of the left side of the yolk sac. 

 A conspicuous red spot marks the position of the heart. 



Spontaneous movements (exercise movements) now occur. 

 These consist of jerking the head from side to side ; wriggling ; 

 reversal of the laterally curved position of the body by turning 

 over ; swimming movements in which the embryo butts against 

 the envelope and subsides ; swimming in a circle. Embryos 

 removed from the capsule at this stage make practically the same 

 movements ; they are unable to progress in a straight line and 

 incapable of prolonged swimming movements. 



Hatching. — Embryos collected when about three weeks old 

 and kept in the laboratory at a temperature averaging about 13 

 C, began to hatch on October if about six weeks after the eggs 

 were probably laid. Nearly all were hatched by October 24. At 

 this time the average length was about 25 mm. The appearance 

 of the embryos just previous to the hatching is shown by Fig. 8. 



Before the hatching period, the envelopes become much sof- 

 tened and considerably enlarged by the absorption of water, mak- 

 ing room for the growing embryo. The latter sometimes escapes 

 by pushing its way through the envelope, leaving a small round 

 hole ; sometimes apparently by tearing or bursting the envelope 

 by means of its wriggling movements. 



