586 EXPERIMENTAL FISH EMBRYOLOGY 



Stage 22. Broad "Neck Strap"; 5.1 mm. (Fig. 20) 



The pericardial membrane has split as far hack as the posterior third of the eye, 

 exhibiting a broad, vascularized "neck strap". The appearance of sclerotomes is here ac- 

 centuated by concentrations of small melanophores in the fascial tissue around them. Cu- 

 taneous melanophores are more numerous, sometimes present in the caudal fin rays. 



Stage 23. Fin Rays in Dorsal Fin; 6.1 mm. 



The "neck strap" (pericardial membrane) has been reduced to about one-half the width 

 of the eye, and Is situated back of the posterior margin of the eye. Fin rays begin to 

 appear in the dorsal fin. Large cutaneous melanophores are thickly scattered over the en- 

 tire embryo. The yolk sac begins to show a rapid reduction in size, measuring 1 mm. In 

 diameter. It is noteworthy that the yolk sac begins to Involute at about the same time 

 that the pericardial membranes are in the process of accelerated regression. 



Embryos at this stage, if removed from their mothers, will feed readily on small 

 Daphnia. 



Stage 2^. "Neck Strap" Breaking Down; 6.5 mm. 



The "neck strap" may be completely broken down at this stage, but it Is sometimes 

 present as a narrow band of tissue. The general shape of the embryo is determined by the 

 condition of the "neck strap," the cephalic flexure straightening as the head lifts up in- 

 to the main body axis. The melanophores In the dorsal head region are stellate and more 

 closely packed. 



Stage 25. Pre-Partur it ion; 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. l87 and 195, where the adult pattern (Induced 

 by the gene Sp for spotting and St for stippling) is composed of large masses of macro- 

 melanophores and mi cromelanopores . Nor can these two types of melanophores be distin- 

 guished. 



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 Cuvler drain 

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



Growth proceeds rapidly and within 2l+ hours after birth the young fry reach an aver- 

 age length, of 8.7 dmi- 



Rate of Development 



In order to obtain some estimation of the developmental rate in Platypoecilus macu- 

 latua, 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 in- 

 terval (as determined by Hopper, 19'<-5). Morphological age for each embryo is established 



