CLEAVAGE IN THE EGG OF CEREBRATULUS. 13 



the fundamental problem of cleavage-physiology. In the following 

 pages I shall briefly deal with the relation between cleavage 

 pattern and " egg- organization " and the nature of cleavage factors 

 viewed in the light of recent experimental studies and especially 

 of the results described in the present paper. 



In order to get at the cause of cleavage pattern we must 

 first look into the present status of the question of germinal locali- 

 zation. Detailed discussion on this subject, however, will be 

 deferred until some other opportunity. 



The idea of germinal localization in the egg was, as is well 

 known, a logical extension oF the organ-forming regions in the 

 blastoderm of the chick to the one-cell stage (His, '74 p. 19). 

 This hypothesis soon gained ground from indirect evidence obtained 

 fiom various experiments upon the egg of ctenophore, frog, etc., 

 and the conclusion was reached that there must be some invisible 

 localization of specialized materials in addition to the promor- 

 phology of the egg. Meanwhile forms were found, in which the 

 egg has a definite visible granular localization, such as Myzosloma, 

 Patella, Dentalium, Ascidians. In the egg of Strongylocentrotus 

 [Paracentrolus) a pigment belt was discovered, which was supposed 

 to correspond in some way or other to the internal zonal arrange- 

 ment of specialized materials. Then it came to be thought 

 that the visible localization of granules, if present, is identical, 

 or approximately so, with the supposed germinal localization. 



This conclusion was refuted by some authors on the ground 

 that the variation in the granular localization does not show corre- 

 sponding difference in the morphogeny, for instance, by Lillie for 

 Unio ('01, p. 262) and by zur Strassen for Ascaris ('06, j). ^^^)' 

 Garbowski found that the pigment ring around the egg of fSlron- 

 gylocenirotiis may take various angles to the embryo axis ('05, p. 603). 



