588 



EXPERIMENTAL FISH EMBRYOLOGY 



THE NORMAL DEVELOPMENT OF THE TELEOST FISH 



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

 brook-trout (Salmo fario) and the rainbow-trout (Salmo Irldaeus) 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 (eplboly). The aemi-gel mediiom which contains the blastoderm cells 

 is bound to the yolk by an encircling gel layer (Lewis, 19'+3)' 



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

 of the "Eandring" or genn 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 eplboly 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. 



Oppenheimer (I956) has eonfinned the earlier statement by Morgan (I895) that the 

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

 concentrated Holtfreter's solution to find that if the excision Is made prior to the 52- 

 cell stage hyperblastulae developed but after that stage, embryonic structures and fre- 

 quently embryos were differentiated. Devillers (19'+T) cultured the small pike egg in 

 5 times Holtfreter's solution beginning at the blastula stage to secure definitive em- 

 bryonic structures. The trout egg (Salmo fario) did not give comparable results, indi- 

 cating ontogenetic stage and differentiation may not be the same in different species. 

 Oppenheimer (19*+?) believes that the yolk has a physical relationship to the early morpho- 

 genetic movements which must not be minimized. 



Representation in the cleaving 

 blastoderm of the embryonic 

 areas mapped out in t)ie gastru- 

 la. The long axis is here repre- 

 sented by the second plane of 

 cleavage. 



The nervous system is indicated 

 by vertical liatchlng, notochord 

 by heavy stipple, endoderm by 

 light stipple, and mesoderm by 

 horizontal hatching. 



16 K 



ji.1.1 



Bl-l-l 



^ ^^.1.1.2.5 



Notochord and endoderv 



Hsrvous ayoton posterior to stldbrsln 



Hesodera, aildbraln to ?d eomlto 



Endoderm, neeodera 



Mesodern posterior to 2d soKlto 



Nervous e/stea and Dssoder«, Torsbraln 



Posterior nesodem 



Mesodern and «xtra-est)i7ordo aeabruw 



Notocbord and erdodera 



Norvoufl Bjsten posterior to aldbraln 



Hesoders, midbrain to 2d somite 



Endoderm, mesoderm 



Mesoderm posterior to 2d somite 



RorrouB systsm and assoderm, forabrmlo 



Posterior mesoderm 



Mesoderm and extrs-embrronlo msmbrmne 



Oerm-rlng •- tall-bud blastema 



Ejctra-embiyonlc membrane 



Germ- ring 



Sxtra-embrjoolc membrane 



Sxtra-eBbryonlc eofflbrans 



Germ- ring 



Extra -embryonic meabranv 



Germ-ring •• tall-bud blastema 



£ztra-eBbT7onlc membrane 



Germ- ring 



£j(tra-embi7otilc membrane 



Extm-eabryoMlc membrane 



Germ- rl Tig 



Extnt-embrjonlc membrane 



CELL LINEAGE OF FUNDULUS. SCHEME FOR THE LOCATION OF MATERIALS 

 IN THE BLASTODERM WHEN THE FIRST CLEAVAGE IS TRANSVERSE 



(From Oppenheimer 1956: Jour, Exp. Zool. 75:'+05) 



