No. I.] CAR-PIKE AND STURGEON. 19 



condition of the periblast (of Teleosts) is most nearly repre- 

 sented. In this regard he would call especial attention to the 

 segmentation stages immediately succeeding this, for it seems 

 to him evident that the marginal cells undergo a metamorphosis 

 similar to that concerned in the formation of periblast which 

 H. V. Wilson 1 has figured. 



After the sixth segmentation continued divisions result in 

 the production of a flattened cell cap (PI. I, Fig. 9), irregular 

 in surface and outline. Its marginal cells are irregular and 

 inconspicuous, and their lateral angles obscure. The cell-cap 

 increasing by continued horizontal division comes next to 

 present an irregular but prominent summit ; its margin is 

 irregular and slightly depressed. At a later stage (PI. I, 

 Fig. 11), the marginal depression has become converted into a 

 moat-like groove. Cells are still seen prominently along its 

 sides although at the summit of the cell-cap continued divisions 

 have rendered them so small that their outlines can be dis- 

 cerned only by means of sections. The noteworthy point, 

 however, at this stage is that at certain points of the outer- 

 most cell-ring the cell-walls are found to have entirely dis- 

 appeared and the nuclei are seen to have entered into the 

 peripheral yolk. This change will at once be seen to corre- 

 spond closely to that in Serramts which Wilson has figured in 

 his memoir, Figs. 21 and 22. At the same time, furthermore, 

 a median vertical section of this stage (PI. II, Fig. 28), indi- 

 cates that it is not at the periphery of the cell-cap alone that 

 this modification occurs ; the entire floor underlying the cell- 

 cap is now smoothly and distinctly differentiated ; it is no 

 longer filled with the rough and half detached cell outlines of 

 an early stage ; the nuclei scattered through it are seen in 

 continued division, occupying a deeper and deeper plane in the 

 subjacent yolk, and budding off at the surface additions to the 

 cell-cap only at irregular points. The cell-cap is now seen to 

 have a thickness of four cells, and their loose clustering has 

 given rise to an irregular segmentation cavity. 



1 H. V. Wilson, On the Embryology of the Sea Bass, Serramts atrarius. U. S. 

 F. C. Bull., No. IX, 1S91. 



