PLATE XXXVI. 



Fig. 1. Ctenolabras. One hour afler 32-ceU stage. [p6 = peribla-st). Out of a large number of 



(4) caps (20) of this stage, thi.s one alone .show.s decided evidence of the fii'st beginning of the endo- 

 derm. The cap is two to three and four cells deep in the central portion, becoming two and 

 then one cell thick at the edge. 



The upper layer of cells is not dotted, and the nuclei are colored, for sake of distinction, more 

 deeply than those of underlying cells. There are considerable intercellular spaces as seen in 

 undotted portions, which are probably due to the cells having been slightly disturbed, or to 

 action of reagents. Numerous vacuole-like spaces (round) varying from size of nucleus to 

 much smaller, are seen prevailingly in the ectoderm. 



Histological evidence of a differentiation of endoderm from edge of cap: — 



1. The edge-cells are stained with carmine — ^not browned and the nuclei are all stained 

 with carmine, with only faint traces of browning; while the remaining cells are more or less 

 deeply browned and the nuclei also more browned, and less brilliant than those of endoderm. 



2. The radial arrangement of protoplasm is much more accentuated in the endoderm-cells 

 than in the ectoderm, and the cells are rather more coarsely granular. We have here the best 

 kind of evidence that the endoderm arises late, from the edge of the cap — thus from what may 

 be regarded as the vegetative portion of cap. The endoderm is in process of differentiation, 

 only certain cells having advanced so far as to be entirely outside the cap and with the charac- 

 teristic absence of distinct cell-boundaries. 



Four cells {en 1,2,3, 4) have advanced to the syncytial stage — representing four free nuclei 

 in the thickened rim of the pellicle (ph), around which the radial hues are very clear and strong. 

 Most of the nuclei appear to be in a condition that precedes the formation of [mitotic] figures. 

 These four cells are wholly within the pellicle, and hence beneath the level of the outer ecto- 

 derm-cells. 



The cells marked en are cells destined to become like en 1-4, but have not lost completely, 

 except in limited portions, their definite boundaries. Many of them are not sharply defined 

 against each other, or against the pellicle, with which they are continuous. 



ec en i is a cell still in the outer rim of ectoderm, but it loses the sharp outline and blends with 

 the pellicle and with the endoderm-cell at its right. 



ec en 2 is another similar cell, which still preserves its outline, but which lies in same level as en 1 . 



ec en 3 still holds its position in the ring of cells en, but it is not more than very faintly and 

 imperfectly delimited from the pellicle. 



We have here, then, the outer ring of cells of cap in process of becoming fused with the pel- 

 licle, only four cells of which have become undoubted syncytial cells. 

 Fig. 2. About one hour after 32-cell stage, from above, treated in same manner and on same 



(16 1 slide. Here the cells are about half the size of those in Fig. 3, indicating that a single divi- 

 sion has occurred. 



Here are seen the same coarsely granular marginal cells in floor of cap, but we see that they 

 have divided tangentially, so that we have here two cells where we before had only one. The 

 outer row of cells is more coarsely granular than the inner row, and their nuclei are at a little 

 distance from the dark margin of the cap proper. 



Now I am inclined to take this outer row of cells as representing the first formed endoderm- 

 cells; still it is possible that the inner row is also endodermic. If the outer row alone is endo- 

 derm, then in Fig. 3, we should have to say that the endoderm has not yet separated from cap, 

 but simply appears as a more coarsely granular part of the marginal cells. In this case 

 Fig. 2 would correspond in age to Fig. 1, which has also about twenty cells in the marginal 

 endoderm. 

 Fig. 3. Ctenolabrus. Portion of cap about 30 minutes after 32-cell stage. From below. 



(15) Stained with osmic only, [ec = ectoderm; en = endoderm; pb = periblast]. 



There are about twenty cells in the periphery of the cap, forming the marginal cells of its floor 

 and projecting a little beyond the smoothly outlined superficial cells. These cells are every- 

 where sharply defined except against the pellicle. Their boundary line against the pellicle 

 is tolerably distinct but not smooth. It is more or less ragged as if the delimitation was not 

 complete. 



