592 



EXPERIMENTAL FISH EMBRYOLOGY 



The axis of the second cleavage plane tends to coincide with the longitudinal axis 

 of the embryo. Cleavage, In general, tends to be geometric and so predictable that early 

 cell lineage studies are possible. Teleost cleavage Is essentially the non-determinate 

 type. There are variations in the qualitative distribution of parts of the blastodlsc so 

 that Oppenhelmer (I956) says: "This lack of constancy of relationship between particular 

 cells of the cleaving blastoderm and specific parts of the embryo is, of course, strictly 

 compatible with the fact that the development of the teleost is of the Inductive type." 



Pasteels (I936) and Oppenhelmer (I936) have shown, by means of vital staining, that 

 the fate map of Salmo and Fundulus are not unlike those for the Amphibia; that the periph- 

 ery of the blastoderm furnishes the mesoderm for the embryo; that the prospective endoderm 

 is found in the cellular blastoderm rather than in the yolky portion of the egg; that the 

 prospective chorda lies in a crescentlc area (Fundulus) Just anterior to the prospective 

 endoderm; and that the prospective nervous system lies Just anterior to the chorda-meso- 

 derm. Gastrulation does not Involve differential mitosis but rather (as In the Amphibia) 

 a re-arrangement of the cells, the types of movement being described as involution, ex- 

 tension, and convergence. The initial expansion of the blastoderm is considered as epl- 

 bolic movement. 



It seems apparent from the work of Morgan (I895), Lewis (I912), Hoadley (I928), 

 Nicholas and Oppenhelmer {l'^k-2) and Tung (19^*^5) that the early blaatodlac can adjust to 

 the removal of single or groups of blastomerea, and still give rise to normal embryos. 

 This suggests that there is no qualitative division in the early stages of Fundulus, and 

 that all regions are totipotent. It now appears that all parts of the trout blaatula are 

 equlpotent (Luther, 1957) and, when the blastoderm of the blastula stage is quartered, 

 each gives rise to tissues and organs (in tissue culture or transplantations) representing 

 all parts of the embryo. 



Gastrulation is the time when there is qualitative segregation of areas, and Oppen- 

 helmer (1956) and Luther (1955) have shown that the dorsal lip of the blastopore functions 

 In teleosts much as it does In the amphibia. Secondary embryos can be Induced by hetero- 

 topic transplantations of pieces of the dorsal lip. Oppenhelmer (l9'+7) believes that the 

 Fundulus dorsal lip shows regional determination, supported by the studies of Eakln (1959) 

 in Salmo. Transplantations to the extra- embryonic areas stand a better chance of surviv- 

 ing and differentiating than when placed In the embryonic shield. 



EPIBOLY OF FISH BLASTODERM 



EXMNSON OF BLASTODERM 



AMMAL POLE 



VE0ET4L POLE 



flNIMAL POL£ VIEW 

 GASTRULATION 1 APPEARANCE Of GERM RING 



EXTRA EMBRYONIC neMBRANE 



SIDE VEW rOLK PLUG 



OeVELOPMENT OF EMBRYONIC SHIELD AND EMBRYO 



(REDRAWN FROM OPFEfCCIMCR I9KI 



