No. I.] 



GAR-PIKE AND STURGEON. 



17 



however, as seen in the figure, it diverges somewhat, tending 

 to appear meridional. In depth and lateral extension its fur- 

 rows are entirely similar to those of earlier cleavage. The 

 position of the nuclei in this stage is shown in PL II, Fig. 24. 

 It will be noted that the blastomeres are now more widely 

 separated by the first and second furrows j these interstices are 

 the first indications of the segmentation cavity. A view of 

 this stage as seen from the side is given in PI. I, Fig. 5. 



FoitrtJi Cleavage (PI. I, Fig. 6) is again a vertical one ; it is 

 in general parallel to the second furrow, resembling in other 

 regards the third cleavage. The extent of 

 the cleavage fissures may be made out in 

 PI. II, Fig. 25, a section in which several 

 nuclei are to be seen dividing for the fifth 

 cleavage. During this stage many varia- 

 tions occur in the size and shape of the 

 blastomeres ; they may readily be reduced, 

 however, to the normal plan of segmenta- 

 tion. A vertical section of this stage (PI. 

 II, Fig. 26) shows the depth of the furrows, 

 and indicates as well the relation of blasto- 

 disc to yolk. 



Fifth Cleavage (PI. I, Fig. 7), results in 

 a normal stage of thirty-two cells : it is 

 carried out often with wide variations, and 

 the lineage of the blastomeres is to be 

 followed only with difficulty. By serial 

 sections of the more regular examples of 

 cleavage in the late i6-cell stage, the nu- 

 clear figures (as for example those of PI. II, 

 Fig. 25) permit, however, the ideal plan 

 of the fifth cleavage to be understood. 

 This the writer expresses in the accompanying figure (Fig. 3), 

 in which is indicated the lineage of eight, sixteen, and thirty- 

 two blastomeres. Those derived first from the four original 

 blastomeres are denoted by a, those from their derivatives by 

 aa, those from their derivatives by aaa ; a second derivative 

 from the original blastomere is denoted by b, a third by c, and 



Fig. 3. 



