temperatures below 15~C development proceeds very slowly Newcombe (1941 42) 

 in an examination of egg cases from three different oyster grounds from July 7 to 

 August 8 in the vicinity of York River observed that a high proportion of the egg 

 cases contained hatching drills throughout this period- The highest percentage of 

 hatching drills, 62% ; was taken on August 8 . 



Cole's (1942) controlled laboratory experiments demonstrate the marked 

 relationship of temperature and the duration of the incubation period. The close 

 similarity of the duration of the incubation period at a relatively constant tempera- 

 ture of 18.3 in the laboratory and at a fluctuating temperature range of 13.5-19°C 

 in the field is equally striking, In addition his field data suggest that development 

 in English waters proceeds at a lower temperature than in the waters of the 

 middle eastern United States (compare with Haskin, 1935; Stauber, 1943: and 

 Federighi, 1931c). 



When considered from the standpoint of completeness of observations and 

 relation of these to fairly typical ecological conditions, the work of Cole (1942), 

 Haskin (1935), and Federighi (1931c) seems to most clearly describe the duration 

 of the incubation period in these regions . More carefully controlled experiments 

 on incubation in different geographic regions are suggested by these studies, 

 many of which, though important in a preliminary way, are quite incomplete . 



The prehatched shelled stages of the drill are known as protoconchs and 

 the hatched stages as conchs . Pope (1910-11) described the emergence of proto- 

 conchs in considerable detail . Prior to hatching, the orifice of the egg capsule 

 is closed securely by the thin operculum . The first protoconch to emerge pushes 

 it outward and others follow. Then for a brief period the newly hatched conchs 

 cling to the sides of the parent case. No one has observed whether the young drill 

 cuts the operculum open with its radula or whether by hatching time the periphery 

 of the operculum has been freed by some action such as bacterial activity. Proto- 

 conchs can also effect their escape from the egg case by drilling . In the laboratory 

 Pope watched a number of young drills cut small circular holes the diameter of a 

 cambric needle and push their way out through this, leaving warty protuberances on 

 the case at the site of escape - Pope believes this mode of escape occurs when 

 protoconchs near the orifice either obstruct the passage or are not developed 

 sufficiently to emerge . The interval for all drills to hatch from an egg capsule 

 or cluster of capsules varies considerably because of the uneven development of 

 the embryos, and may extend from four to 38 days (Pope, 1910-11). 



The degree of winter survival of young drills hatching from the late 

 summer and autumn wave of oviposition in northern waters has never been 

 determined. Stauber (1943)states that it is quite doubtful that there is any 



39 



