4 BIOLOGICAL LECTURES. 



as made up of a host of elementary ultra-microscopical corpus- 

 cles ("pangens," " biophores," etc.), specifically different, capable 

 of assimilation, growth, and multiplication, and arising by divi- 

 sion of preexisting bodies of like kind. Developed as a purely 

 theoretical hypothesis, and within somewhat narrower limits, by 

 Darwin, this conception was expanded, and brought into more 

 direct relation with observed fact, especially by De Vries and 

 Wiesner, who showed how the assumption of such elementary 

 self-propagating corpuscles at the basis of living matter enabled 

 us to bring all the observed phenomena of genetic continuity 

 under a common point of view. The fundamental hypothesis 

 itself — i.e., the genetic continuity of the ultimate morphological 

 units — has, however, always remained, and still remains, a pure 

 assumption, incapable of direct proof or disproof ; for, with the 

 exception of Altmann and a few of his followers, all are agreed 

 that such elementary corpuscles, if they exist, must lie be- 

 yond the limits of microscopical vision. Altmann, however, 

 has sought to identify the elementary units, or "bioblasts," 

 with the visible protoplasmic granules; and, in his writings, the 

 series of Latin aphorisms initiated by Redi culminates in the 

 saying, onine granidum e granido (!), but this conclusion has not 

 been taken very seriously by most other investigators. 



I have given this very brief sketch of the theoretical side 

 of the question merely as an introduction, and shall dwell no 

 farther on it at this point, since my main purpose is to ask 

 attention to the visible, as opposed to the hypothetical invisible, 

 structure of protoplasm. A subject so vas|, displaying so great 

 a conflict of opinion, must be very briefly treated within the 

 limits of a single lecture ; and I shall, therefore, confine the 

 discussion in the main to the protoplasm of the echinoderm- 

 egg, which is accessible to every one, has been made a classical 

 object through the studies of such leaders of research as Flem- 

 ming, Biitschli, and Hertwig, and illustrates as clearly, perhaps, 

 as any other the various interpretations of protoplasmic struc- 

 ture that have been given. 



In thin sections of well-preserved material, the protoplasm 

 of a star-fish or sea-urchin Qg% gives the appearance, under a 

 high power, of a fine meshwork or framework composed of innu- 



