DURATION OF DEVELOPMENT 



The length of time required by pilchard eggs to develop to the hatch- 

 ing stage after being spavmed and fertilized was roughly deterrainud by 

 placing pilchard eggs taken in plankton samples into hatching jars aboard 

 the vessel while at sea and following their de-velopraent. This was done 

 during several cruises in I9UO. In some sanplds the development of eggs 

 v;as followed from the blastodermal cap stage to hatching. From these 

 experiments, it was concluded that the pilchard egg ordinarily required 

 about three days to develop to the hatching stage from the time of spawn- 

 ing. Because the temperature of the hatching- jars could not be kept 

 constant vifith the facilities available at sea, it fluctuated several 

 degrees during each twenty-four hour period. Hence, the data derived 

 from these experiments is not of direct use in the study of the influence 

 of temperature on the rate of development. 



Since the pilchard egg is spawned only during a fevr hours of the 

 twenty-four (8:00 p.m. to midnight), and since it requires several days 

 for the process of development, a plankton sample should contain eggs 

 in various stages of development j furthermore, there should be a sharp 

 Separation of the stages among eggs derived from each day's spawning. 

 A tabulation of the stages of devjlcpment present in the samples (table 6) 

 reveals that a sample typically contains stages in various degrees of 

 development separated by gaps of one or two stages, which are taken to 

 represent the time gaps between successive spavmings. Thus, \rc have a 

 basis for separating the stages present in a sample into their respective 

 days of spawning^/; with the least advanced stage (or stages) taken to 

 represent the most recent spawning, and so on. In some samples one or 

 more days of spawning may not bo represented, but this can usually be 

 ascertained by a comparison with other samples. As a consequence, there 

 are often insixfficient landmarks to indicate the age of such stages as 

 are present. 



Having effected a separation of the eggs in a plankton sample into 

 their respective days of spawning, the actual time of development of each 

 stage present in a sample can be determined. The midpoint of spawning, 

 10:00 p.m., is taken as the hour when development commenced. The time 

 of preservation of each plankton sample marks the endpoint of development 

 of the eggs present in the sample, and this time was recorded as a part 

 of the routine observations during collection of the material. From these 

 data it is possible to derive the probable length of time, in hours, 

 that each of the several stages in most samples had been developing. 



WATHK TEI-IPERATURE3 



In correlating the development of pilchard eggs with the temperature 

 of the water at the station of collection, it is necessary to express the 



£/ Separation into days is indicated for each sample listed in table 6. 



138 



