192 PAUL S. WELCH. 



within. The presence of the plugs indicates that no emergence 

 has yet occurred. These plugs vary in form in different cocoons 

 but are usually cone-shaped at both ends, the outer end extending 

 beyond the distal margin of the neck and the inner end extending 

 into the cavity of the cocoon. 



The number of eggs included in a cocoon is subject to con- 

 siderable variation. In more than five hundred cocoons studied 

 the maximum was found to be seventeen and the minimum one. 

 Cocoons containing only one egg were rare, but numbers above 

 ten were rather common, although the general average was 

 somewhat lower. Dixon (1915, p. 85) found a maximum of only 

 thirteen or fourteen. The size of the cocoon is somewhat 

 dependent upon the number of eggs present. 



DEVELOPMENT OF THE EGGS. 



Owing to the transparency of the cocoon, development of the 

 eggs can be followed with some ease. Two distinct steps occur 

 in the progress from the newly formed egg to the free living 

 condition of the resulting worm, namely, escape from the egg 

 membrane, and subsequent emergence from the cocoon. Or- 

 dinarily, eggs "hatch"- in a very few days by breaking the 

 delicate, transparent egg membrane but escape from the cocoon 

 is subject to surprising variation and since this phenomenon is 

 directly concerned with the problems in hand, special attention 

 was given to it. This variation occurs not only among different 

 cocoons but also among eggs of the same cocoon. Emergence 

 ordinarily occurs during the tenth to the twentieth day but not 

 infrequently the sojourn within is much longer. Delayed emer- 

 gences are usually, if not entirely, due to some inability to pass 

 through the neck of the cocoon and usually results in the death of 

 the worm, although occasionally escape is finally accomplished. 



Dixon (1915, p. 86) was unable to find more than nine young 

 developing from one cocoon and thought it probable that in 

 those with an unusually large number of eggs certain eggs only 

 develop, the others ultimately deteriorating. Attention was 

 given to this matter and the occasional failure of certain eggs to 

 develop was confirmed. However, more than nine worms 



