EXPERIMENTAL FISH EMBRYOLOGY 58? 



by comparlaon with the graded series of stages. Chronological age represents the actual 

 developmental rate for each stage. 



The theoretical age of all the memhers of a hrood was determined hy recording the 

 date of birth of a previous brood. This is based upon the fact that fertilization of a 

 successive complement of eggs within a gravid female takes place on about the seventh day 

 after the birth of its previous brood ( Happer, 19^+3 )• Theoretically then, the embryos 

 carried by a gravid female, which had dropped a brood eight days previously, are 2h hours 

 old. 



This theoretical age value, it must be noted, la only an approximation, since matura- 

 tion and. fertilization of a complement of eggs is spread apparently over a period of two 

 or three days. The seven day interval, as determined by Hopper (19'*'3), has been found to 

 be only an average time lapse. The estimation of the true chronological age may be de- 

 termined by comparing the theoretical and the morphological age values. 



A reliable estimation of the theoretical age was obtained by study of those broods 

 from fully matured females which contained 25 or more embryos, and which had given birth 

 to at least, two previous broods at an interval of approximately 28 days. Only 21 out of 

 55 females examined had these qualifications. Data on many young females were found to 

 be unreliable since many of them had run highly irregular reproductive cycles, varying 

 from 35 to 90 days between broods; and a large percentage of their embryos were dead or 

 abnormal. For these purposes, too, data on exceptionally small embryonic broods (those 

 containing less than 10 embryos) were not considered. 



The chart (Fig. 21) siimmarlzea the data on 21 embryonic broods plotted in the follow- 

 ing manner: Each vertical bar represents all the members of one entire brood carried by a 

 single gravid female platyfish. The length of each vertical bar, projected on the ordi- 

 nate, shows the range of morphological ages found in each embryonic brood. In some cases, 

 especially during the later portion of the gestation period, the embryos are all of a 

 single morphological age; these are represented by plus (+) signs. 



The embryos are divided into theoretical age groups according to the number of days 

 that have elapsed since the birth of the previous brood (leas the seven day interval) and 

 are arranged along the abscissa. Usually there is more than one brood in each age group. 



The mean morphological ages for all the embryos of each theoretical age group are 

 also plotted on the chart, and these values are connected by the dotted line. 



From the chart, it may be seen that there are two kinds of variations. First, there 

 is the wide range of morphological stages among the embryos found within any one gravid 

 female; and second, the variationa of the average morphological age of a brood with re- 

 spect to its theoretical age. 



The greater apparent spread of morphological stages in the earlier broods may be at- 

 tributed to the unequal time lapse between stages distinguished on the basis of morphology 

 alone. 



Using the Information described previoualy on the reproductive cycle of the platy- 

 fish, it was thought that not only a graded series of morphological stages but also a 

 chronological series could be obtained. On the basis of these data, some estimations of 

 the time of development of each stage could have been made. However, the variation, aa 

 demonstrated by the chart, proved to be so great that an estimation of the true chrono- 

 logical age was in^osaible. 



