— i6 — 



A marked advance is indicated by the liverworts, mosses 

 and ferns in which the original germ cell forms an 8-celled 

 embryo by division in three planes each at right angles to the 

 other two, the group of four cells on either side of the first 

 cleavage plane then developing in the linear manner. The 

 formation of an 8-celled embryo is not in line with previous 

 developmental processes in the plants, and it may be interpreted 

 as the independent and spontaneous appearance in the plants 

 of a developmental feature normal to animals. 



A second marked advance in seen in the phanerogams in 

 which a multicellular embryo with dilferentiated outer and 

 inner cells develops into a continuous series of more or less 

 distinct units (phytons) which collectively form the plant. This 

 is a further departure from the normal type of plant develop- 

 ment in the direction of the animals, especially in the direction 

 of the cœlenterates which from a comparable embryo build up 

 by continuous budding comparable arborescent structures. 



The two great advances in the vegetable kingdom may 

 therefore be interpreted as resulting from the assumption by 

 the plants of developmental features preeminently characteristic 

 of animals. 



The unicellular animals, being compact and with a marked 

 tendency to radial growth and development, logically find their 

 first multicellular expression in a radially symmetrical embryo 

 which develops into a radially symmetrical adult either by 

 a sort of thallophytic mass increase as in the sponges or by 

 the formation of a definite and unified body as in the cœlen- 

 terates. 



/ The logical, but inflexible, sponge and cœlenterate types 

 led to nothing further. Advance was through a third type 

 represented by the acœlomate animals in which the typical 

 animal form and structure is profoundly modified by the 

 assumption of a number of features normally characteristic 

 of plants. 



In the acœlomate animals the body form and the various 

 organs and tissues are very variously developed and there is 

 no vascular or respiratory system. With the crystallization 

 of the so-called cœlome and the appearance of a vascular and 

 a respiratory system facilitating the repair of waste and leading 

 to a marked increase in bodily activity, in other words the 



