6 bulletin: museum of comparative zoology. 



After expulsion from the body of the worm the eggs float near the 

 surface, Avhere they are immediately fertilized. The eggs are quite 

 large; measurements of the embryos in the 16-cell stage gave the 

 diameter 0.36 mm. The segmentation is total and unequal. Four 

 large yolk-laden macromeres are cut otf from the four smaller yolkless 

 micromeres. These latter then divide repeatedly and overlap the four 

 macromei'es, and thus the gastula is formed by epibole. Although my 

 observations are far too incomplete for anything but general conclusions, 

 it appears that the early stages of the segmentation are strikingly 

 similar to those of Xereis as described and figured by Wilson ('92). 



Figure 5, Plate 1, represents an embryo in the 16-cell stage, which 

 occurs about three hours after extrusion into the water. It will be seen 

 that the large macromeres are heavily laden with deutoplasm-spheres, 

 while the protoplasm of the micromeres is finely and uniformly vacuo- 

 lated, giving the appearance, when seen in sections, of a delicate network. 

 The centrosomes are of large size and stain quite deeply in heematoxyliii. 



Figure 6 represents the condition of an embryo 9^ hours old in which 

 the blastopore (bp) is just about to be closed. It will be seen that a 

 distinct segmentation cavity (sgc) makes its tippearance at this stage. 

 This cavity may, however, be due to the action of reagents, and may 

 not represent the natural condition. Unfortunately, all of my material 

 having been killed in Perenyi's fluid, I am unable to make any state- 

 ments concerning this point. It will be noticed that some of the 

 micromeres at this stage are beginning to exhibit large intracellular 

 vacuoles. This is especially true of those cells about 180° away from 

 the blastopore, and also of some in the immediate vicinity of the 

 blastopore. In later stages this vacuolization aff'ects all of the cells of 

 the embryo, both those of the ectoderm and entoderm, and it is certainly 

 true that for the first week of its life the larva owes its increase in size 

 almost entirely to the remarkable development of intracellular and 

 intercellular vacuoles. In this connection it is interesting to note that 

 Davenport ('97) has shown that in the case of tadpoles the early growth 

 is almost entirely due to tlie imbibition of water. Soon after this, when 

 the embryo is about 9^ hours old the blastopore closes, and the large 

 deutoplasm-laden cells are completely enclosed by the micromeres. The 

 embryos then become uniformly ciliated and swim about witli consider- 

 able rapidity. 



Figure 14, Plate 2, represents an embryo 24 hours old. Two eye- 

 spots are now beginning to appear, and between these there is a col- 

 lection of greenish-colored cells. These cells stain very deeply in 



