4 T.H.MORGAN. 



Tanystylum and Phoxichilidium. 



The eggs of Tanystylum measure .08 mm. in diameter (in pre- 

 served specimens), and those of Phoxichilidium .05 mm. In 

 both animals there is a regular segmentation. In Plate III, 

 Figure a is a surface view of an egg of Phoxichilidium divided into 

 two equal parts ; in Fig. h of an egg into four, but with the seg- 

 ments shifted around, and Fig. c of an egg into eight equal parts. 

 Similarly Figs, d and e for the two and four-cell stages of 

 Tanystylum. The eight-celled stage I did not obtain for this 

 species. Fig./" is a surface view of an egg of Tanystylum at about 

 the twenty-four-celled stage, and an optical section of an egg at 

 this stage shows each cell to run from the periphery to the center 

 of the egg, where they all come together at a point. Each cell 

 is thus pyramidal in shape and contains a single nucleus. 



In Plate I, Fig. 8, there is a section of an egg at a later stage 

 than the last. This and the following sections are from eggs 

 which were cut in paraffine. As the section is a later stage 

 than the preceding surface view, the pyramids are narrower 

 than before. Fig. 11 shows a somewhat older stage for 

 Phoxichilidium. Yery soon after this — perhaps after the next 

 cleavage — a most important change takes place in the egg. I 

 have not seen the actual change in the living eggs, but serial 

 sections leave not the slightest doubt as to the process. Each 

 nucleus divides radially into two, and this is followed by division 

 of each pyramid into two parts in a plane at right angles to the 

 radius of the circle lying in that pyramid. This is shown by 

 Plate I, Fig. 9, for Tanystylum. Here it is seen that by a 

 process of multipolar delamination the egg divides into two 

 germ-layers — an outer peripheral circle of cells and an inner 

 mass of cells. These latter soon round off and leave no trace of 

 the pyramidal arrangement. In Fig. 9 one cell is still seen 

 running from the periphery to the center. This section does not 

 show a nucleus in the inner part. The inner cells I shall speak 

 of as the entoblast and the outer circle as the ectoblast. This 

 figure (9) is somewhat diagrammatic, inasmuch as only part of 

 the inner cells is shown, for usually they are more closely packed 

 together than here shown ; and in this case it is due to a part of 

 the section having broken away (redrawn in figure) and set free 

 some of the inner cells. In Fig. 10 is a more accurate drawing 



