Origin of Deuterogenesis 89 



net result of growth must be expansion in all directions from that 

 point as indicated by the arrows. For simplicity I represent three 

 such areas of expansion by the three pairs of arrows. Now let it 

 be supposed that the original arrangement of the forces involved 

 be such that when a certain size has been attained (as in the 

 description of the invaginating gastrula of Amphioxus discussed 

 in the earlier part of this essay) invagination must occur, then as 

 a result, the proliferating areas are doubled upon themselves, 

 Fig. 40 B, and at the point of doubling the resultant effect must 

 be a radial accumulation of cells and the consequent destruction of 

 the radial symmetry, and the origin of growth in length (accom- 

 panied by at any rate partial closure of the blastopore), which will 

 be continued so long as the margins of folding retain their power 

 of production of undifferentiated cells. This is shown to be so in 

 the diagram by the direction of growth which before invagination 

 has a counterbalancing effect and so the status quo or radial 

 symmetry is retained, but, as soon as an antagonism is converted 

 into a coroperation and the balance is upset, the blastopore must 

 tend to close and on the accumulation of radially arranged material 

 growth in length ensues. That is to say, deuterogenesis is as in- 

 evitable a consequence of the doubling of the proliferating area as 

 the doubling is, on my hypothesis, an inevitable result of the original 

 general structure of the egg and it requires no special determinant 

 to call it into being. 



I confess that with regard to the origin of the deuterogenetic 

 centre in Amniota it is not so simple a matter. In Amniota, as 

 I believe, there is no true blastopore. In a Rabbit for instance the 

 deuterogenetic centre seems to arise "of its own accord." For the 

 moment the problem must remain unsolved, but further observa- 

 tion and reflection may show it to be as inevitably a consequence 

 of what has gone before, as it is in the case of gastrulas with 

 typical blastopore. 



Clearly this explanation of the origin of the deuterogenetic 

 centre, is, if correct, applicable to all gastrulae formed by in- 

 vagination, or indeed, all that have a distinct blastoporic lip. If 

 an organism retains a radial symmetry after gastrulation, it must 

 do so by the dying out of the deuterogenetic centre of cell pro- 

 liferation. By a retention of one part, and suppression of another 



