22 Ilyhridizalion of Echinoids. 



cleavage of the egg proceeds, there is a gradual diminution in size 

 of the spherules, until in the gastrula all trace of them has been lost. 



My conclusion regarding these spherules is that they are droplets 

 of fat. In all of the sea-urchin eggs that I have been able to study 

 by adequate methods I have found that in the clear protoplasmic 

 matrix there are included, in addition to the so-called active inclu- 

 sions, two tyi3es of inert bodies, fat droplets and yolk plates. The 

 method of fixation used in preparing these Cidaris eggs for study is 

 not adequate for the demonstration of the yolk plates, nor is it 

 adequate for the demonstration of the fatty nature of the spherules 

 described. These spherules, however, are in the nature of droplets, 

 not platelets, and their distribution is similar to that of undoubted 

 fat droplets in other sea-urchin eggs. Before making these supple- 

 mentary studies I was inclined to the noncommittal description of 

 these bodies as deutoplasmic spherules, but that, I now believe, 

 would be in the nature of a rather unnecessary circumlocution. 



The Cidaris egg is, as I have pointed out, unusual among echinoid 

 eggs. A study of its protoplasm by modern cytological methods 

 should prove of more than usual interest. My material was pre- 

 pared for the study of chromosomes and has been satisfactory for 

 that purpose. It is wholly inadequate for the study of cytoplasmic 

 inclusions. 



The fat droplets and yolk are shifted about readily in the proto- 

 plasm while this is in a sol phase. I have already described the 

 movement of the droplets away from the nucleus during the early 

 stages of division. A movement in the opposite direction begins as 

 the egg passes from the anaphases to the telophases of division. 

 While the chromosomes are developing into the usual chromosomal 

 vesicles (plate 1, figs, c and e; plate 2, figs, f and g; fig. 25), there 

 is a centripetal movement of the fat droplets, so that with the re- 

 formation of the nucleus these again lie distributed uniformly between 

 the wall of the nucleus and the cell wall. 



In some eggs, prior to the appearance of any constriction of the 

 surface of the egg in division, there is a noticeable pressing together 

 of spindle fibers along the line of future cleavage in the cytoplasm, 

 and a very definite impression of internal cleavage is given (fig. b 

 of plate 1). 



The microsomes visible in the surface of the egg, as shown in 

 figures a and b, plate 1, are of a nature differing from that of the 

 fat droplets. They destain much more readily, and it will be noted 

 that they are not shown in any of the remaining sections of the 

 entire eggs. It will also be noted that these are the only sections of 

 straight-fertilized Cidaris eggs shown, and that the other sections 

 are of cross-activated eggs. I have considered very carefully the 

 question of their abundance in one class of eggs and their scarcity in 



