374 FISH FOR EXPERIMENTATION 



Stage 25. Pre-Parturition; 6.9 mm. 



The extra-embryonic membranes and the yolk flanges are absent. The yolk has been re- 

 duced to a mean diameter of . 8 mm. No trace of the adult color pattern is yet visible, 

 there being only a general increase in the number of melanophores on the peripheral 

 areas. This is true even in embryos of Culture Nos. 187 and 195, where the adult pat- 

 tern (induced by the gene Sp for spotting and St for stippling) is composed of large masses 

 of macromelanophores and micromelanopores. Nor can these two types of melanophores 

 be distinguished. 



Stage 26. One Hour after Birth; 7. 9 mm. 



Birth activity begins with a rupture of the fertilization and follicle membranes by the 

 violent movements of the embryos. The embryos break into the ovarian sac and then 

 one by one they are extruded through the oviduct into the water. 



In earlier stages, the heart extends forward from the conus, and the sinus venosus lies 

 directly beneath the tip of the head. As the yolk mass becomes reduced, the heart pivots 

 on the conus and the yolk sac portal system shrinks until the ducts of Cuvier drain 

 directly into the sinus venosus, which eventually moves into place posterior to the conus. 



Growth proceeds rapidly and within 24 hours after birth the young fry reach an average 

 length of 8. 7 mm. 



Rate of Development 



In order to obtain some estimation of the developmental rate in Platypoecilus maculatus, 

 records were kept on the number of embryos and their stages found in each timed gravid 

 female. The morphological age of the embryos was determined by comparing each with 

 the twenty-five established graded stages. 



The following terms are used in this section: Theoretical age is the value determined 

 for the entire embryonic brood from the date of the previous brood, less the seven day 

 interval (as determined by Hopper, 1943). Morphological age for each embryo is estab- 

 lished by comparison with the graded series of stages. Chronological age represents 

 the actual developmental rate for each stage. 



The theoretical age of all the members of a brood was determined by 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 (Hopper, 1943). Theoretically then, the embryos carried by 

 a gravid female, which had dropped a brood eight days previously, are 24 hours old. 



This theoretical age value, it must be noted, is only an approximation, since maturation 

 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 (1943), has been found to 

 be only an average time lapse. The estimation of the true chronological age nnay 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 

 56 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, vary- 

 ing from 36 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. 



