GENERAL CONSIDERATIONS. 29 



calls the division of the protoplasm in TdphrkUum free spore forma- 

 tion, though he confesses that he does not understand the stag-e in 

 which the spores are being cut out, and gives us no conclusive evidence 

 that the substance between the spores is protoplasm. 



Timberlake (1902) has described a process of cleavage in the forma- 

 tion of the swarni-sporcs of Trijdrodlefi/on. lo'fricnhifuin similar to that 

 which I have described. In this alga the protoplasm forms a la3'er of 

 an even thickness around a central vacuole, and this protoplasm is 

 divided into a single layer of spores by narrow furrows cutting from 

 the central vacuole outward and meeting similar furrows from the 

 surfaces. 



The mechanics of this process of division present a ver}'^ perplexing 

 problem. Sections like those shown in PI. II, tig. 8, and PI. Ill, fig. 10, 

 where cleavageisonly partly complete, have an appearance that suggests 

 the eft'ect of cracking on the surface due to drying. If a colloidal sphere 

 were allowed to dry by evaporation from its surface, it would crack 

 and split in a manner nuich like the sporangia of RJilzopiis. That 

 such an explanation is not adequate for this cleavage phenomenon is 

 clearly evident from the fact that the furrows are tilled with cell sap 

 in living specimens throughout the entire process of division. One 

 can scarceh' imagine any bodj" cracking from dr} ing out when the 

 crevices are tilled with a watery liquid. In any case such an explana- 

 tion would not account for cleavage by angles cutting out from vacu- 

 oles embedded in the protoplasm. 



An explanation that would in a measure account for the angles being 

 pushed out from the vacuoles is that the vacuoles take up water from 

 the surrounding cytoplasm In' osmosis through their membranes, 

 which would cause an outward pressure against the latter. If now cer- 

 tain parts of this membrane should become weaker than other parts, 

 these weaker parts would be pushed out by the internal pressure. 

 Such an explanation, however, would not account for the surface fur- 

 rows, as they are not surrounded on all sides ])y an osmotic membrane, 

 there being no membrane across the mouth of the furrow at the periph- 

 ery of the sporangium. (PL II, figs. 8 and 9.) If such a membrane 

 be present, it is so thin that it is not visible with the highest powers 

 of the microscope, and hence it is doubtful whether it would be more 

 resistant to outward pressure from within the furrow than the plasma- 

 membrane and cytoplasm at the inner edge of the furrow. 



That the plasma-membrane and vacuolar membrane should possess 

 sufiicient rigidity to cut into the protoplasm after the fashion of a 

 knife is entirely foreign to our conception of these membranes. 



The most probable explanation the writer has found for the mechanics 

 of the cleavage is on the basis of local contractions of the cj^toplasm, 

 somewhat comparable to the phenomena exhibited in the naked proto- 

 plasm of amoebae and pseudopodia. In PI. VI, figs. 28-31, the writer 



