266 INEZ WHIPPLE WILDER. 



the embryo begins to make spontaneous movements, forcibly 

 bending the free anterior and posterior ends of the body to right 

 and left. By this time the limb buds have appeared, and the 

 dorsal surface of the embryo assumes a decidedly grayish hue, 

 owing to the beginning of pigmentation, which rapidly increases 

 as development goes on, and takes the form of a decided pattern 

 by the twenty-fourth day. 



External gill bushes may be readily distinguished as early as 

 the sixteenth day. On the twenty-fifth day, however, neither 

 the mouth nor the gill slits have opened to the exterior. On the 

 thirtieth day the mouth has opened by two lateral slits but is 

 still closed in the middle region. At this time, also, the two most 

 anterior gill slits upon each side are open, the more anterior being 

 just posterior to the gular fold. 



A considerable accumulation of liquid early appears about the 

 embryo, and continues to increase in amount, distending the 

 egg envelopes, which are thus placed under a constantly increas- 

 ing tension. From this cause it becomes increasingly easier to 

 remove the embryo from the egg membranes, until by the eigh- 

 teenth day a puncture of the membranes is followed by a rapid 

 expulsion of the embryo through the force of the escaping fluid. 

 This is undoubtedly a preparation for the easy rupture of the 

 membranes and escape of the embryo when the time for hatching 

 arrives, the fluid serving meanwhile to equalize the external 

 pressure upon the delicate embryonal tissues and prevent injury 

 to them. 



Thirty days is the longest time for which the continuous de- 

 velopment of any batch of eggs has been reported (H. H. Wilder, 

 '99), so that as to the exact duration of the period of development 

 within the egg I have very limited data. It is seldom that one 

 knows the exact date when a given batch of eggs is deposited, or, 

 knowing this date, can carry on the observations successfully 

 under artificial conditions to the time of hatching. Hilton 

 states that the rapidity of development depends upon humidity 

 and temperature, but gives no data whatever concerning the 

 duration of the whole period. Wilder describes the thirty-day 

 embryo above referred to as a fully developed larva in all respects 

 except the still large yolk mass. "The pigmentation was com- 



