Protogenesis and Deuterogenesis 83 



one another, and yet immediately on separation (which separation 

 is by no means always nor even perhaps ever immediately absolute) 

 appear to attract one another strongly. 



But such a state of things is not unknown in other analogous 

 phenomena in which attraction and repulsion form the visible 

 effects of the internal force. In a bar magnet there are strong 

 force "centres" of the two poles, while midway between the poles 

 there is but little attractive force. Break the magnet and im- 

 mediately strong attraction centres occur near the point where 

 before there was none or but little, even if the broken ends are in 

 loose contact. I do not mean to compare the supposed form of 

 energy in any way with magnetism except by analogy; for 

 magnetism is always bipolar while the form of energy under 

 consideration appears to be automatically unipolar and bipolar 

 alternately. 



PART II 



If the account given here of the gastrulation of Amphioxus is 

 correct, it will be realised that the formation of the gut cavity and 

 the definitive ectoderm and endoderm at any rate of the anterior 

 part of the future animal is the direct result of and indeed part of 

 segmentation. The product is on the whole a radially symmetrical 

 animal. It is what I called in earlier papers on the Rabbit the 

 result of the primary centre of growth (and later by paraphrase 

 protogenesis). It is to be sharply distinguished from growth in 

 length, which by the origin of a secondary growth-centre system 

 (deuterogenesis) round the lips of the blastopore, converts the 

 radially symmetrical organism into a bilaterally symmetrical one 

 by adding on a metamerically segmented trunk and tail. In a 

 similar way I attempted to show that the archenteron and 

 definitive ectoderm and endoderm are formed as a direct result of 

 protogenesis in the Amphibian, v. " The Growth in Length of the 

 Frog Embryo." 



The whole process of formation of the gastrula with its archen- 

 teron is regarded as being the simple working of the driving power, 

 so to speak, of this supposed form of energy guided by the 



62 



