nSH FOR EXPERIMENTATION 



377 



FUNDULUS HETEROCLITUS 



I A B C D I 



a 2 a 





^iill ^iWigllJiBii^J 



The typical cleavage pattern of the egg of 

 Fiindiilus heteroclitus. Schematic. A repre- 

 sents the uncleaved blastoderm, B the two- 

 cell stage, C the four-cell stage, D the 

 eight-cell stage and E the sixteen-stage. 

 F and G show the direction of the fifth and 

 sixth planes of cleavage in the marginal 

 cells of the sixteen-cell stage; these planes 

 are not represented in the central cells of 

 the sixteen-cell stage because they are here 

 horizontal and difficult to follow. 



A, B, C, D and E are drawn at approxi- 

 mately the same scale; the magnification 

 is greater for F and G. The position of 

 the first four planes of cleavage is indicated 

 by numerals; see text for explanation of the 

 designation of cells by letter and number. 



(From Oppenheimer 1936: Jour. Exp. Zool. 73:405) 



Diagrams illustrating the results of experiments 

 in which stained blastomeres were followed 

 through to gastrulation. The figures marked 

 "A" in each case represent the blastoderms 

 immediately after staining those morked "B" 

 and "C" are the later stages of the same 

 embryos. 



The relation of the first or second cleavage 

 plane to the embryonic axis may be deter- 

 mined. Occasionally the embryonic axis is 

 oblique to the early cleavage planes. 



Usually two of the 8-cell stage blastomeres or 

 four of the marginal cells of the 160 -cell stage 

 blastoderm give rise to the early embryonic 

 shield, and (in one instance) two of the 16 -cell 

 stage blastomeres formed the entire shield. 

 The germ ring is formed by peripheral cells of 

 the 32-cell stage and the material for the fore- 

 brain comes from the central cells of the 16- 

 cell stage. 



