16 T. H. MORGAN. 



stage and the next which I have figured there is a gap. In this 

 next section, Fig. 4, we see the number of nuclei at the upper 

 pole to be more numerous than before and much smaller. The 

 protoplasmic cap has become larger and its protoplasm is for 

 the most part without the amoeboid processes, of the last figure. 

 A very important change has taken place between these two 

 stages, viz. the formation of an inner layer of cells. Within the 

 area of the cap appear a few somewhat flattened cells in Fig. 4, 

 and these send out processes into the underlying yolk. Two of 

 these cells are shown in this figure. I have not made out any 

 definite arrangement for these cells, but they seem to lie only 

 under the upper surface of the embryo. Where these cells come 

 from or at what time they appear must in part be a matter of 

 conjecture, but much light is thrown on their possible origin by a 

 study of somewhat older stages. In Fig. 5 is a section of such a 

 stage. The number of nuclei in the outer germ-layer has nearly 

 doubled, and the area itself covers a much greater surface of the 

 egg than in the last stage. Under the blastoderm are seen five 

 inner cells with their pseudopodial extension. The larger num- 

 ber of these cells here than in the last figure is due in part to 

 the greater thickness of the sections. At the lower pole are 

 two nuclei. These belong to the outer germ-layer, although they 

 show (as did all the outer at an earlier stage) the protoplasmic 

 extensions into the yolk. This section itself throws little light 

 on the cells of the inner layer, but in other sections at a similar 

 stage I have found what seems to furnish the solution of the 

 problem. From such sections I have drawn Figs. 6 and 7. 

 These are taken from the periphery of the cap of cells at some 

 such point as P in Fig. 5. Fig. 6 shows two cells which have 

 just separated from the outer layer of cells, and we also see 

 beyond them a single mass of protoplasm with two nuclei which 

 presumably have just divided. I have not seen any karyoki- 

 netic figures in nuclei dividing in this direction, and it is possible 

 we have here a direct division, as Heider has recently shown in 

 Hydrophilus at a similar stage of development, although it is 

 equally as possible it is due to poor preserving agents. Again, 

 in Fig. 7, we see a single mass of protoplasm with two nuclei, 

 the result of division. These I believe to give a clue to the 

 origin of the inner cells of the preceding figures, and to point 

 out that they too have had a similar origin from the outer layer. 



