THE SPIRAL TYPE OF CLEAVAGE. 



\1 



Fig. 3. — Two-cell stage of a fresh-water nemertean 

 after the attainment of the resting stage, showing 

 the flattening together of the cells and the tem- 

 porary intercellular cavity. 



the blastomeres would appear distinctly disadvantageous, for 

 here a much larger portion of each blastomere is in contact 

 with others than in the looser cell aggregate, so that respiration 

 and excretion apparently cannot occur as freely as in a loose 

 mass of rounded cells. The periodic appearance of the large 

 intercellular cavities filled with liquid, as described by Kofoid 

 ('95) for Limax, and which 

 occur in other forms as well 

 (Figs. 3 and 4), is probably 

 a provision for aiding the 

 excretion, some such provi- 

 sion having become neces- 

 sary under certain conditions 

 (fresh-water or terrestrial life 

 in moist localities), in conse- 

 quence of the extremely inti- 

 mate unionof theblastomeres. 



Moreover, we do find that 

 in many forms, and espe- 

 cially in those in which the 

 spiral type of cleavage oc- 

 curs, there is a distinct 

 condensation of the proc- 

 ess of development, which 

 finds expression in the 

 early appearance of dif- 

 ferentiation and morpho- 

 genesis for forms in which 

 any such tendency existed ; 

 it must be clear that a con- 

 formation to the principle 

 of minimal contact sur- 

 faces among the blastomeres will afford the most direct and 

 intimate communication possible between them. 



It is then, I believe, on account of the necessity for just this 

 intimate communication between the component parts of the 

 organism that the spiral cleavage, which in its simplest form 

 fulfills these conditions in the most perfect manner possible, 



Fig, 



4. — Four-cell stage of a fresh-water nemertean, 

 showing the temporary intercellular cavity. 



