526 BERTRAM G. SMITH 



are pink with blood. The tail is rapidly increasing in size and 

 beginning to straighten out. The front limb rudiments now 

 take the form of conspicuous outstanding lobes, the distal ends 

 not yet divided into digits. Toward the close of this stage the 

 rudiments of the hind limbs appear. The entire dorsal surface 

 of the body is well sprinkled with pigment cells; the eyes are 

 becoming deeply pigmented. The lateial line system is well 

 developed. During this stage occurs the rapid formation of 

 the gular folds. 



The lateral vascular bands no longer appear as such, but on 

 their site are differentiated the two main trunks of the vitelline 

 veins (figs. 193 and 194). As compared with the earliest position 

 of the lateral vascular bands, the vitelline veins lie considerably 

 nearer the ventral surface of the yolk sac. While the vitelline 

 system of veins is primarily a paired one, almost from the begin- 

 ning one side is usually found better developed than the other. 

 The heart now contracts at the rate of about forty to sixty beats 

 per minute, and the blood pulsates regularly through the vitelline 

 veins. 



The cilia are especially well developed on the external gills; 

 here the ciliary currents are strongest. Spontaneous muscular 

 movements now occur at frequent intervals. The movements 

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

 of the laterally curved position of the body by turning over; 

 swimming movements by means of which the embryo butts 

 against the envelope; and swimming in a circle. The functional 

 value of these movements seems to be to afford exercise for the 

 developing muscles. Embryos removed from the capsules at 

 this stage make practically the same movements; they are unable 

 to progress in a straight line and are incapable of prolonged swim- 

 ming movements. 



During the later stages of development before hatching, the 

 water in which the embryos are kept has a pronounced 'fishy' 

 odor. 



Stage 23: {figs. 195, 196, 260 and 261). The limits of this 

 stage are fixed to include the time of hatching. In a given lot 

 of embryos hatching does not take place all at once, but extends 



