File:Biological lectures delivered at the Marine Biological Laboratory of Wood's Holl (sic) (1899) (20380332505).jpg

Title: Biological lectures delivered at the Marine Biological Laboratory of Wood's Holl (sic)
Identifier: biologicallectur1899mari (find matches)
Year: 1899 (1890s)
Authors: Marine Biological Laboratory (Woods Hole, Mass. )
Subjects: Biology
Publisher: Boston, Ginn & Co
Contributing Library: MBLWHOI Library
Digitizing Sponsor: MBLWHOI Library

View Book Page: Book Viewer
About This Book: Catalog Entry
View All Images: All Images From Book
Click here to view book online to see this illustration in context in a browseable online version of this book.

Text Appearing Before Image:
'
Text Appearing After Image:
THE SPIRAL TYPE OF CLEAVAGE. 235 upon fixed material, but since these were made a very inter- esting series of observations upon living cells have appeared (G. F. Andrews, '97; A. E. Andrews, '97a, '97b, '98a, '98b, '980), which must, I believe, lead us to the conclusion that intercel- lular protoplasmic continuity between animal cells is at least of very wide occurrence, if not universal, during developmental stages (Figs, i and 2). This being the case, we have, it seems to me, very strong evidence upon both the botanical and zoologi- cal side for the correctness of the so-called organism stand- point, as distinguished from any cell theory. And I be- lieve we must admit that intercellular • protoplasmic continuity is of the greatest importance in our interpreta- tion of the organism. ... , . r Fig. 2. — Two-cell stage of the egg of a fresh-water AltnOUgll my study 01 nemertean, showitig the ectoplasmic activity in cleavage was beo'Un from a ^^ cleavage furrow and the rounded form of the ^ ... cells immediately after division. point of view coinciding fully with the cell theory, my observations have led me to the belief that any explanation of the spiral cleavage is impossible upon that basis. The developing egg of the turbellarian, annelid, and mollusk is, I believe, more than a cell mosaic; it is dis- tinctly an organism at every stage. I desire, then, to discuss the spiral type of cleavage, not as a series of self-differentiations or cellular interactions, but as a series of processes, each of which has its cause in the organism, as distinguished from this or that cell. From this point of view alone, as I believe, is it possible to explain the various phenom- ena observed, and especially the lack of correspondence in ori- gin and fate of the blastomeres, which is not explicable by any strictly cellular theory of development. In his classical paper upon Nereis, Wilson ('92) has given us a most valuable discussion of the origin of the spiral form of cleavage, and it is desirable at this point to review this briefly. In general, his view is as follows : " The ficndainental forms of cleavage are primarily due to mechattical co7iditions, and are only

Note About Images