8 BIOLOGICAL LECTURES. 



that were referred to at the outset. I shall take up in order 

 the questions raised at page 5. 



The Nature of the MesJiwork. — Although in earlier papers 

 I was inclined to regard the meshwork of the echinoderm-egg 

 as a reticulum, further studies have left no doubt whatever, in 

 my opinion, that in the resting cell it is in reality an alveolar 

 structure — or, as I do not hesitate to call it, an emulsion — 

 such as Biitschli has described. I was first led to this conclu- 

 sion through the study of sections of the eggs of sea-urchins 

 ( Toxopneiistes) and star-fish (Asterias) ; but whatever doubt may 

 have remained was completely dissipated by the study of the 

 living eggs of Asterias (Fig. i, b), Echinarachniiis, Arbacia, 

 Ophiiira (Fig. 2, a), under high powers. All of these eggs give 

 in life essentially the same appearance, though no two are 

 exactly alike. In all, the protoplasm consists of innumerable 

 closely crowded minute spheres suspended in a clear basis. 

 The spheres may be called the alveolar spheres, or more briefly 

 the alveoli, though strictly speaking the latter term should 

 designate the cavities which the spheres fill. The clear basis 

 in which they lie, and which forms the inter-alveolar walls, may, 

 with Mrs. Andrews, be called the contimioiis substance. Scat- 

 tered about in these walls are numerous granules, or microso^nes, 

 far smaller than the alveoli, which often give the appearance of 

 an irregular network. If now we compare these appearances 

 of the living protoplasm with those seen in the sections mounted 

 in balsam, we find at first sight very considerable differences. 

 More critical study shows, however, that the differences are 

 almost wholly due to the effect of differential staining and to 

 the difference of refractive index in the mounting media in the 

 two cases. The alveoli of the living protoplasm form the spaces 

 of the meshwork. The latter consists of the continuous sub- 

 stance with the granules suspended in it. In the section, what 

 especially strikes the eye is the meshwork ; for the alveolar 

 spheres do not stain, and their contours become indistinct in 

 the highly refracting balsam, while the continuous substance 

 stains slightly, and the granules intensely, thus giving the 

 appearance of a conspicuous granular meshwork. We thus 

 arrive at a definite answer to two of the questions propounded 



