THE STRUCTURE OF PROTOPLASM. 



on page 5, namely : (i) the meshwork shown in sections is not 

 a network, but the expression of an alveolar or emulsion-struc- 

 ture, and (2) proper fixation does not produce a mass of coagu- 

 lation-artifacts, but preserves the visible structure very nearly 

 as it exists in life. 



The above conclusions are based mainly on the study of star- 

 fish eggs, but are confirmed by the facts observed in other forms. 

 In Arbacia the emulsion is considerably finer, the alveoli meas- 

 uring on an average no 

 more than i.o micron, 

 while the finer granules 

 are relatively less nu- 

 merous. The pigment- 

 granules characteristic 

 of this form appear to be 

 nothing other than mod- 

 ified alveolar spheres. 

 In Toxopneustes the al- 

 veoli measure approxi- 

 mately from i.o to 1.3 

 microns, while the gran- b 

 ules are more numerous 

 than in Asterias. In 

 Echinarachnius the al- 

 veoli are less uniform 

 in size than in Asterias, 

 the largest measuring 

 up to about 1.7 microns, 

 while the granules are 

 less numerous. The 

 egg of Ophiura, finally, has an extremely coarse structure, the 

 alveolar spheres measuring on an average 3.0 to 4.0 microns, 

 while the granules or microsomes are also very large and, in the 

 superficial layers of the protoplasm, even more numerous than 

 in Toxopneustes. The protoplasm of Ophiura (Fig. 2) is highly 

 favorable for study, not only on account of the great size of its 

 elements, but also by reason of the remarkable fact that these 

 elements are colored in life, the alveolar spheres being in most 



FIG. 2. (a) Protoplasm from a living ophiuran egg (Ophi- 

 ura), slightly compressed, so as to spread the yolk- 

 spheres somewhat apart ; (t>) the same as seen in a 

 section (sublimate-acetic, iron-ha:matoxylin ; 1200 diam- 

 eters). 



