HARPER: BINARY FISSION AND SURFACE TENSION 155 



The second difference lies in the very fundamental fact that, as 

 noted, in Volvox the germ cell grows to relatively large size before 

 dividing and the daughter cells grow in size between the successive 

 cell divisions. This is a very long step toward the full metaphytic 

 habit in ontogeny. It marks a return to the habit of the simple 

 protophyte like the bacteria and the appearance of a new point of 

 departure in the development of the morphogenesis of a metaphytic 

 plant body out of the primitive habit of reproduction by swarm- 

 spores which is seen in Chlamydomonas and Sphaerella. In these 

 protophytes, the cell having reached maturity forms from four to 

 eight swarmspores by rather rapidly succeeding divisions of the mother 

 cell. They escape by breaking of the mother cell wall and then as 

 free individuals proceed to grow to the size of the parent. 



Swarmspore formation in Chlamydomonas and Sphaerella is a step 

 beyond the conditions in Euglena, for example, where in ordinary 

 reproduction each cell division is followed at once by the individualiza- 

 tion of the daughter cells and their independence as separate organisms. 

 In these particulars we may distinguish three steps in the evolution 

 of the metaphyte from the typical protophyte. 



1 . Cell division, in simple, direct alternation with growth, reproduction 



and individualization, practically simultaneous and identical 

 processes. Euglena. 



2. Cell divisions at unequal intervals, reproduction multiple and in 



alternation with growth. Individualization delayed. Chlamy- 

 domonas, Sphaerella. 



3. Cell division and growth in direct alternation, reproduction multiple 



and individualization delayed by intercalation of a true em- 

 bryonic period. Volvox. 



In Volvox individualization is already in essence the complex 

 process of differentiation and maturing which we find in the highest 

 plants and animals. In Gonium individualization of the daughter 

 colony, as I have shown in a former paper ('12), is accompanied merely 

 by certain gliding movements of the cells upon each other by which 

 an approximation to a least surface configuration is achieved so far 

 as is possible for sixteen ovoid cells arranged in a flat plate. 



In Volvox, with the retention of multiple or colony reproduction 

 as in Sphaerella, we have growth intercalated again between each 

 successive cell division and also a specialization in function between 

 germ and somatic cells. Complete individualization is delayed till 

 the colony has become very many-celled. What may be called a 

 pseudo-growth comparable to the elongation of the cells just back of 

 the root tip by absorption of water and the formation of large central 

 vacuoles is also represented in Volvox by the formation from the cell 



