FISH FOR EXPERIMENTATION 375 



The chart (Fig. 21 on page 383) summarizes the data on 21 embryonic broods plotted in 

 the following 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 ordinate, shows the range of morphological ages found in each embry9nic 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 (less 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 variations of the average morphological age of a brood with 

 respect 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 morphol- 

 ogy alone. 



Using the information described previously on the reproductive cycle of the platyfish, it 

 was thought that not only a graded series of morphological stages but also a chronolog- 

 ical 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, as demon- 

 strated by the chart, proved to be so great that an estimation of the true chronological 

 age was impossible. 



i. The Normal Telecost: Fundulus Heteroclitus 



Within the last decade a considerable volume of research has been published on the 

 brook-trout (Salmo fario) and the rainbow-trout (Salmo iridaeus) in Europe; on Oryzias 

 latipes in Japan and the United States; and finally, on Fundulus in this country. Before 

 proceeding to experimental techniques it is in order that we describe, in summary form, 

 some of the characteristic features in the development of the Teleost egg. 



The egg consists of a large mass of fluid yolk surmounted by a disc of protoplasm, both 

 contained within a plasma membrane and protected by a heavy chorion. The embryo is 

 derived from the blastodisc which alone divides, ultimately to form a sheet of cells 

 which will encompass the yolk (epiboly). The semi-gel medium which contains the blas- 

 toderin cells is bound to the yolk by an encircling gel layer (Lewis, 1943). 



With expansion of the blastoderm there is a thinning of its center and a thickening of the 

 "Randring" or germ ring around its periphery. From a specific point on this germ ring 

 the thickened embryonic-shield extends toward the center of the blastodermic area. With 

 continued epiboly of the circumferential germ ring, there is an interruption at a point 

 near the origin of the embryonic -shield, and an infolding (involution) of cells to form the 

 endodermal roof of the archenteron. Differentiation of the embryonic-shield into brain, 

 optic and otic vesicles, and somites then occurs. 



Oppenheii-ner (1936) has confirmed the earlier statement by Morgan (1893) that the (Fun- 

 dulus) embryo could survive without its yolk, by explanting the blastoderm alone into 

 Holtfreter's solution to find that if the excision is made prior to the 32-cell stage hyper- 

 blastulae developed but after that stage, embryonic structures and frequently embryos 



